Cisgender came into use among transgender activists and academics in the 1990s. The Oxford English Dictionary added it in 2015. It works the way "heterosexual" does for sexual orientation: it describes the majority group rather than leaving it unmarked as normal.
Naming cisgender identity makes it easier to talk about privilege. Cisgender people can usually use a public restroom, show an identity document or seek medical care without having their gender questioned. These ordinary advantages are often invisible to the people who have them, in the same way white privilege can be.
Some critics reject the word, arguing it is unnecessary or political. Its use is about accuracy. A word for the majority makes clear that being cisgender is one way to be, not the standard every other gender identity is measured against. The shorthand "cis" is common.