Dropping is a slang term that means to debut or to become available for purchase.
A drop is the release of one or more NFTs for purchase at a particular time and place.
The phrase drop implies that there will be strong demand for the release - therefore, it's often used by in marketing or promotional materials to make a release sound more exciting.
Curated NFT platforms like Nifty Gateway, MakersPlace, and KnownOrigin have regular drops several times a week (or even several times a day!), in which one or more artists release a new NFT collection for purchase. Releasing these at a regular, advertised time helps drive excitement and demand for the work and for the platforms themselves. To add to the excitement, some NFTs are only available for purchase at the time of the drop - so if you miss the drop, you've lost your chance to buy directly from the creator!
The term drop comes from hip-hop culture, where it's used to hype the release of new albums. The phrase has since been appropriated by the mainstream music industry (and other industries) to refer to the release of any album, song, artistic, or commercial release.