What does it mean that girls always send text messages asking me what I'm doing?

She just wants to talk to you. It's like saying hello and hi to you

This is not supervision, but a way to open a chatterbox.

You seldom contact others. She is satisfied with you, so she doesn't want to lose contact with you.

So if you want to develop further, talk to others first and keep in touch.

Asking what you are doing is also a way to care about you. If you don't contact her when I miss you, she will contact you.