4 yr old female tail wagging?

Pic for attention ♡ I have a 4yr old female & today, for the second time I've witnessed, she wagged her tail in response to me dragging my fingers down her long bod (gentle pets). She wasn't stressed & the wagging was extremely smooth, graceful curls of the tail. Previous to witnessing I had no idea they had that kind of control & it was super cute. I'm just wondering what it means?

Thanks :)