the tippet is the final end of the tapered leader that the fly is tied to.
What everyone else has said so far is correct. I will expand on this a little more for you and go into a little more detail. As was stated, the final end of a leader, the end to which the fly is tied, is called the tippet. Generally speaking, leaders are tapered, starting larger in diameter where connected to the fly line and then gradually getting smaller until the very end portion, the narrowest in diameter portion, called the tippet. The tippet is, once again, the portion of the leader to which a fly is tied. You will also read/hear of people adding tippet. They are referring to tippet material which comes on a spool and is tied to the tippet end of the leader to either a) lengthen the leader; or, b) to replace tippet material that has been removed through cutting off flies and replacing them with other flies. The article at this link will provide a good explanation for you:
https://www.hatchmag.com/articles/understanding-leaders-and-tippets/7711248