<< Comment tu t'appelles?>> translates to "What is your name?"
The direct translation is "how do you call yourself?" because the word <<comment>> means "how," and the verb <<s'appeler>> is "to be called." This is a reflexive verb, meaning the action of the verb reflects back onto the subject. For an English example, you could say "He washes himself," or "She dresses herself." This is reflexive, because it is done to oneself. If you directly translate <<tu t'appeles>> you get "you call yourself." Put that in the context of the whole sentence, "how do you call yourself," but in English we would just say "What is your name?"
This sentence uses <<tu>> which is the informal version of "you," whereas the formal version would use <<vous.>> This means that you would use this sentence while speaking to someone you knew well or in a casual setting, like a friend, or meeting someone in a bar. Typically, when you meet someone in a more formal setting, you should use the formal version of "you" so as not to come off rude. If this were the case you would say:
<<Comment vous vous appelez?>>
I hope this was helpful!