Many people confuse the words "what does that mean" and "what is that supposed to mean." Although they are grammatically the same, these phrases have some different semantic meanings. "What does that mean?" is a question about a general topic or idea, while "what is that supposed to mean?" is more specific and usually implies that you want to know what someone else means by their statement.
When to use that
The word that is a demonstrative pronoun and takes the place of a noun or verb phrase when you are referring to something that has already been mentioned. It also joins clauses in a sentence to show which thing you are talking about.
When to use which
Which is used after verbs, nouns, and adjectives to identify a person or thing that you are talking about. It often replaces the genitive form of a noun, such as which or whose:
The word that can also be used to refer to an era or period. It is a good alternative to using phrases such as the last time or that day. It can also be used to show that you are referring to the same event as another person: