The t–v distinction is about how people address each other in different languages. In some languages, the word used to address someone depends on whether that person is a friend or someone in authority, such as a teacher or a boss.
For example, in German, if you want to talk to a friend, you would use "du" (pronounced "doo"). But if you're talking to someone who is your teacher or boss, you would use "Sie" (pronounced "zee").
This is the t–v distinction, where "t" stands for the word you use when you talk to friends and "v" stands for the word you use when you talk to people in authority.