Tsundere is a term originating from Japan. It is derived from the terms tsun tsun (ツンツン), meaning to turn away in disgust, and dere dere (デレデレ) meaning to become ‘lovey dovey’.
It essentially means that a character goes from a cold and distant (sometimes even hostile) attitude to a more friendlier and warmer attitude, at times falling in love as well.
What I have seen so far in regards of Tsundere characters is that a character like this always stays a little stubborn and/or hesitant in the way they appreciate and show their love for another character/protagonist/player.
Par example: they are frowning and blushing whilst telling the player in a huffy whisper that they would like to go with them to the carnival. All the while they do not dare to make eye contact and they cross their arms. All of this body language tells the reader/watcher/player that they are insecure (avoiding eye contact) and defensive and distant (crossing arms). Yet the blushing tells everyone that they are sincere in their wish to spend time with the other character.
I believe, though am not certain, that there are a lot of Animes which have this Tsundere character. And I bet that if you would type tsundere in the Youtube search machine, you will no doubt find some scenes where Tsundere characters are shown. So if the term still mystifies you, you could always look it up there.