Cisgender is one of many terms coined to describe one's gender identity. Cisgender refers to people whose gender identity matches how they were identified at birth.
So if you were born and they called you a boy, and you are now an adult and you identify as a man, then your gender identity might be referred to as cisgender.
Several people cite Carol Buijs, a transsexual man, as the first person to coin the term cisgender to describe people who are not trans. In her entry on cisgender, Julia Serano points out that the terms "trans" and "cis" are commonly used in science, where "'trans' means 'across' or 'on the opposite side of,' whereas 'cis' means 'on the same side of.'"
The term is used in a similar way in the context of gender, where someone whose experience involves crossing from one gender to another is "trans" and someone whose experience is of being on the one side is "cis".