Some mares you'll never notice, others are HUGE flirts ;)
For those who show signs (to a stallion, gelding, even another mare), they will lift their tails, possibly squat, and "pee" ... usually just a little bit, and their vulvas will "wink" They may also make noises (kinda a squeal).
It's called being in season, being in heat, being "in" or in estrus if you want to be technical ;)
Jessi
MareStare cam!
www.marestare.com/cheyaut.php
www.CheyAutRanch.com