When to Pap Smear? What is it?

I am 21 and sexually actively but have never had a gynaecologist appointment in my life. My most recent doctors visit (unrelated issue) was two weeks ago and he mentioned that I am at the age to have my first Pap smear? What exactly does that involve and how important is it to do?

I asked my mom and she said it isn’t important and that it is a really uncomfortable and invasive procedure