Cisgender people are people whose internal knowledge of their gender is the same as the gender people thought they were when they were born. “Cis” is a shorter word meaning cisgender.
A cisgender woman lives as a woman today, and was assumed to be female when she was born. A cisgender man lives as a man today, and was assumed to be male when he was born.
Many cisgender people never think of themselves as cisgender because this matching of gender identity and expectations from birth is so common.
For more information on transgender people and the trans movement, check out the National Center for Transgender Equality.