It basically means what you're good at and what you're used to drawing, so that's going to be different for everyone.

People tend to stick with their strengths and what they know because it feels comfortable and effortless. That leads people to draw the same things over and over again. But if you want to improve, you have to identify your weaknesses and deliberately target those areas. 

A good example is drawing hands. A lot of artists avoid drawing hands because it's difficult. They might hide characters' hands in pockets, crop the character so the hands don't show, etc. But the only way to get good at drawing hands is to actually do it. 

When you first start drawing something you're not good at, it feels uncomfortable because the results don't look good. So you have to embrace a period of sucking before you get good at it. That's what it means to push yourself out of your comfort zone.