<aside> 💡 TL;DR. Onchain means the art and metadata are on the blockchain. Most NFTs are NOT actually fully onchain - rather the NFT itself is stored on IPFS or an external server and the contract that points to the art is onchain.
Onchain storage is preferable because it means that users can verify all facets of the NFT. However, very few NFT projects opt for this method of storage as it’s expensive.
The reason for this is simple— images contain a lot of data, especially when those images exist in collections in the thousands.