A girl kept staring at me

So I’m a student a university. Today we had a lecture and I sat behind a one girl and her friend. I have been hanging out with them lately so I know them a little.

So, this girl and her friend kept turning her head to look at me and instantly turned back when I looked at them.

Can you explain what does it mean, I just don’t understand. Is that a thing I will understand in 20 years while I shower or what