Most simply, a cisgender person is anyone who is not transgender. A cisgender person fits into one of the two gender categories of girl/woman/female or boy/man/male and only one of those categories.
The word was created mainly to end confusion in transgender conversations about who to refer to all the various groups of people. Some trans activists believe having only a word, "transgender" to refer only to the minority implies the group is more deviant (or "queeer" if you will) whereas giving a name to both sides lends itself to equality. The term "heterosexual" formed in a similar way.