A Combo is a combination of two or more NFTs. Combos are also NFTs.
You can mint pretty much any Combo you can imagine. You can change your CryptoPunk's gender, give your Bored Ape kung-fu powers and build an "All-Star NBA Team" bundle NFT with your favorite players. Essentially, you can transform your NFT from an asset in your portfolio into a piece of art you co-created.
Above, we can see that the BAYC NFT was combined with the Clown Hat NFT to create a new Combo NFT.

Combo Collection

Combo Collections, organize Combos made up of the same Collections and Add-ons into categories. Each Combo Collection has a "Mint Rule" that needs to be satisfied before a Combo can fall under it. Once a Combo is minted into a 1Combo Combo Collection, it automatically appears on OpenSea under the same Combo Collection name.
For a Combo to belong to the "Decorated Ape Collection", for example, it would have to include at least one NFT from the BAYC Collection and one NFT from the Accessory Collection. When a Combo Collection's rule is satisfied, the newly minted Combo becomes part of that Combo Collection.