Being cisgender means identifying as the gender one is assigned at birth.
Gender is often assigned on the basis of the appearance of one’s genitals.
“Cisgender” is often considered the opposite of “transgender”. It’s important to use the word “cisgender” instead of the word “normal”, which implies that trans people are abnormal.
To use an example: if I am born with a vagina, I will assumed to be a girl. I will be assigned female. If I identify entirely as a girl/woman/female, I identify as the gender I was assigned at birth, and therefore, I am cisgender. – SF