Tsundere is a common attribute of anime character, which is featured by cold attitude against his/her lover in public but actually really into them in private situation. it commonly came with main heroine with yellow twin-tailed hair.
Note that there are so many variations of Tsundere (e.g. Yandere) but don’t mix up them. Better off waiting another my post to have grip of -dere word family:)
Tsundere often becomes just a Dere in final Epsode or in Season two.