.eth is a TLD for crypto addresses (the system is called ENS). Instead of saying, send it to 0x827DCAB38F00624466A99211e9c5a48b9D6Ec14D or whatever I can say "send it to myname.eth". The ENS is an NFT, so all the NFT infrastructure can be used to manage it. NFT's are generic digital assets so any marketplace can do the swap. The major downside is that gas on eth is SUPER expensive, when I regestered my ENS name, it cost $5 for the name, and $200 for gas. On the other hand, ETH is an obsolete chain (there is a v2 coming, and L2's are starting to come on, but V1 is obsolete... lets be honest), and some new more modern chains solve the problem better. I have a .AVAX name (not fully deployed yet) which cost $15 for the name (which has a weird public notice period thing, kind of interesting) and $.15 for gas. Again, it's an NFT, so I can use any of the numerous marketplaces to buy and sell it.