"Collection Offers" indicate that a buyer has put forth an amount for which he is willing to purchase any NFT from a specific collection.
These offers are sent to every owner of an NFT in the said collection and are not individual offers on specific NFTs.
When you receive a "Collection Offer" you may decide to "Instant Sell" any NFT you own from the collection that received this offer.
NOTE: You cannot counteroffer a collection offer.