ENS is short for Ethereum Name Service. It is a piece of code on the blockchain (a smart contract) that allows you to have a nice human-readable name for your wallet. So instead of a long string of letters and numbers, such as 0x2E70...0576, you can give people a name, such as tchebotarev.eth.
The annual fee for owning an ENS name is $5 USD; however, registering and renewing also require transaction fees, approximately the same as minting an NFT. As of July 2022, it would cost about $15-30 USD in gas fees.
ENS is a great way to establish your identity in a crypto community. ENS is also the most popular decentralized app on Ethereum, considered by many essential to creating a strong identity in web3.
Many services like Sloika rely extensively on making scalable solutions and use ENS name in our naming so that it's less dependent on keeping a separate set of database records.
For the adventurous types, you can actually set up ENS to resolve to an actual, tamper-proof decentralized website.
Note: When you create a username on a platform (ie Opensea), you are dealing with centralized entity, thus increasing the prevalence of old web2 habits in a web3 world.