@BeHappy  Im glad you asked!!

Cis is a short word for Cisgender - which essentailly means that the gender that you are (ie male or female) matches what your body looks like.  So you have a female body and a female gender - or a Male body and a male gender

Cisgender is a newer term, and has emerged to be the 'opposite' of Transgender - which means that your gender is different to your body.

its jsut a word that is easier to use than describing somone as not being transgender.   So we have Transgender and the oppositie Cisgender