So what is this ENS thing anyway?
The Ethereum Name Service (or ENS) is, in their words:
"...a distributed, open, and extensible naming system based on the Ethereum blockchain. ENS’s job is to map human-readable names like ‘alice.eth’ to machine-readable identifiers such as Ethereum addresses, other cryptocurrency addresses, content hashes, and metadata." (source)
In other words, the ENS seeks to be for the decentralized web what DNS is for the traditional internet (and hopefully, not too much like what DNS has been for the traditional internet): an infrastructure through which human-readable names can resolve to machine-readable addresses, including Ethereum addresses.
So what can I do with it?
Well, instead of having to remember that crazy long cryptographic hash address, you can, for a modest fee, reserve a human-readable .eth username. (Or you can splurge a bit if you're looking for an exclusive three-letter address.) Next time you need someone to send you tokens, they can just type your .eth name into MetaMask, and your address should appear:
Keep in mind that, just like traditional Internet domain names, your .eth domain will have to be renewed periodically, otherwise it can be reserved by someone else. Additionally, just like traditional Internet domain names, .eth domains can be reserved, and then traded -- the only difference being that this is the decentralized web, and those .eth domains can be traded as NFTs directly from MetaMask (currently just Mobile) or on marketplaces such as opensea.io.