If you look at the link from samclem, you’ll see it in the coin packages when you scroll down. It’s just a metallic round with a ‘D’ or a ‘P’ or an ‘S’ stamped on it to show which mint the coins came from.
The Mint’s primary purpose, of course, is to make legal-tender coins. But it provides certain other merchandise to that portion of the public which is either coin collectors or investors. Two of these are proof sets and uncirculated sets. A proof set is simply coins struck from a brand-new die on special, highly polished planchets, so that they are the “ideal” of that coin for each coin in the set – as attractive as that particular coin can get. “Proof” of course comes from them being in origin the tests run on each new die/coinage, much like “printer’s proofs” from a publisher. It was discovered these highly attractive coins would fetch a premium, so they were then coined using polished planchets and sold at a premium by the Mint.
Uncirculated sets are just that, brand new uncirculated coins from the general coinage run, peeled out and put in sets. The mint tokens are included because they’re easy and cheap to produce (being essentially penny blanks) and identify the set as being an uncirculated set and from which mint.