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.
Tsundere has a very established line, `Kanchigai Shinai Deyone, Betsuni Anata No Tame Ni Yattanjya naindakaraNE`(Don’t misunderstand. It’s NOT for you! by my wild traslation).
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.