yep! Wrinkles and creases. :) If they're just from squinting or frowning, they wouldn't be that deep. I keep an eye on mine, and there's always a very faint line, but sometimes it's a deep crevice/crease.
Ayurvedic medicine has lots of juicy fun facts like that. Everyone raises their eyebrows and frowns, after all, so how come some people have creases and others faint lines? People with deep horizontal lines on their foreheads are worry-warts.