As in, swinging her legs back and forth over the ground, like pendulums, I believe.

It’s when you’re sitting down and swinging your legs backwards and forwards

Not sure if it’s culturally-specific but I think it’s basically intended as a (likely involuntary) body language display showing interest and excitement. She is so engrossed in the explanation of Naruto (or the individual giving the explanation) that she is not paying attention to what her feet are doing. And what they are doing is acting as an outlet for her excitement.

I couldn’t picture this without seeing her as either very very short or very very young. Don’t grown ups feet reach the floor?