This word is used here to mark a break in the main story line. Here Matthew tells background information about the twelve apostles. (See: [[rc://en/ta/man/translate/writing-background]])
Possible meanings are 1) "the Zealot" is a title that shows that he was part of the group of people who wanted to free the Jewish people from Roman rule. AT: "the patriot" or "the nationalist" or 2) "the Zealot" is a description that shows that he was zealous for God to be honored. AT: "the zealous one" or "the passionate one"