DEFINITION
A 1/1, 1 of 1, or one of one, is an NFT that has been issued as a single, unique edition. One of ones are seen as more scarce and therefore more valuable, because only one person can own them at a time.
CONTEXT
When an NFT is minted, the creator can choose to issue any number of editions of their work. If the creator elects to make multiple editions, these editions will all appear exactly the same, differing only in their edition number and/or token ID.
Issuing multiple editions of an NFT allows more people to collect that nifty. However, the law of supply and demand dictates that the more editions that exist of an NFT, the less valuable each edition becomes.
1/1s therefore have more value than multi-edition NFTs. (By this same logic, 1/1s are generally more valuable than open editions, since open editions are designed to create many editions of that item.)
On the Ethereum network, 1/1s are typically ERC-721 tokens, since the alternative ERC-1155 token standard was created in order to make minting multiple editions of an NFT cheaper and easier.
VIDEO EXPLANATION
For an explanation of 1/1s and other NFT concepts, follow the Niftyist TikTok account.