What Is Marginality
The world isn't the way it is just because that's how things were always meant to be. Everything that exists now was made this way at some point in history.
In our world, some groups of people have more power than others. They have more money, more status, more media that portrays them in a good light, more opportunities, more time, and more respect and credibility in the eyes of the general public. It's not because they're smarter, stronger, wiser, prettier, or better. It's because their ancestors had the stronger weapons and the ruthlessness to take power. Once they had the money and power, they established systems to make it look fair. Hundreds and hundreds of years have passed, and now our culture(s) shrouds these origins. We get taught growing up, through messages sent by the media, schools, and others in our lives, that the systems that control society are how things have always been, that they're how things should be, and we learn ideas and behaviors that reinforce this in our minds.
Those who benefit from these systems are called privileged popover available They didn't do anything good or bad necessarily, but because of their ancestry, their body, or their appearance, they receive benefits in society just for being who they are.
Everyone else is thrown to the margins of society. This is what we call marginalization, to be made disadvantaged in society because of something you can't help. Your race, your gender, your medical history, your heritage, your lover, your accent, or your body. Who you are as a person is stigmatized and there's seemingly no way out. 
It's from here that we get those -isms. Racism, (Cis)Sexism, Ableism, Heterosexism, Sizeism... the list goes on.
Racism privileges white people, and marginalizes people of color, especially black people. This system is also known as White Supremacy.
(Cis)sexism privileges men and masculinity, and marginalizes women, femininity, and gender non-conformity, especially transgender women. This system is also known as Patriarchy. (Or Cispatriarchy, Heteropatriarchy, Cisheteropatriarchy, Heterocispatriarchy, (Cis)Patriarchy, (Cis)(Hetero)Patriarchy, etc. etc.)
Ableism privileges the able-bodied tooltip available, and marginalizes anyone with physical and mental disabilities. This is sometimes called Eugenics, though that term usually refers to a more specific sub-part of Ableism.
Heterosexism privileges straight people, and marginalizes same-gender loving people, including people who identify as lesbians, gay, bisexual, or pansexual. This system is sometimes called Homophobia and Biphobia.
Capitalism privileges those with money, and marginalizes those without. It governs our world and while it puts forth the idea that anyone could just get more money, that isn't usually true. The children of the rich start off rich from birth, and so they don't have to do as much to continue being rich. They have connections with other rich people. The rich all tend to benefit from other systems of marginalization as well. The poor are more often born into poorer families and more often are members of other marginalized groups. Members of marginalized groups have a harder time getting by under the rule of Capitalism, and are more likely to become poor and to stay poor, whereas the more privileged in society have better chances at climbing the social ladder and getting more money.
Being marginalized just generally makes life more difficult. You face acts of violence, microaggressions, discrimination, and you're more likely to have low self-esteem and develop mental health issues popover available[2-6]
A note on "Marginality" vs "Marginalization"
If you decide to do more research into these topics, you'll probably notice that more often people use the term "Marginalization" where I've been using "Marginality." I made this choice simply because Marginality is shorter and easier to type over and over, but the terms are interchangable. If you decide to use a search engine to find more articles, I'd suggest searching "marginalization" for more results than "marginality."
 Billson, Janet Mancini. “No Owner Of Soil: The Concept Of Marginality Revisited On Its Sixtieth Birthday.” International Review of Modern Sociology 18, no. 2 (October 1, 1988): 183–204.
 Roots, James. “Political Socialization and Marginalization.” In Politics of Visual Language: Deafness, Language Choice, and Political Socialization, 7–25. McGill-Queen’s Press-MQUP, 1999.
Nadal, Kevin L., Yinglee Wong, Katie E. Griffin, Kristin Davidoff, and Julie Sriken. “The Adverse Impact of Racial Microaggressions on College Students’ Self-Esteem.” Journal of College Student Development 55, no. 5 (2014): 461–74. doi:10.1353/csd.2014.0051.
 Burn, Shawn, Kelly Kadlec, and Ryan Rexer. “Effects of Subtle Heterosexism on Gays, Lesbians, and Bisexuals.” Journal of Homosexuality 49, no. 2 (August 9, 2005): 23–38. doi:10.1300/J082v49n02_02.
Huebner, David M., and Mary C. Davis. “Perceived Antigay Discrimination and Physical Health Outcomes.” Health Psychology 26, no. 5 (September 2007): 627–34. doi:10.1037/0278-618.104.22.1687.
Donovan, Roxanne A., David J. Galban, Ryan K. Grace, Jacqueline K. Bennett, and Shaina Z. Felicié. “Impact of Racial Macro- and Microaggressions in Black Women’s Lives A Preliminary Analysis.” Journal of Black Psychology 39, no. 2 (April 1, 2013): 185–96. doi:10.1177/0095798412443259.