ENS is the acronym for
Ethereum Name Service, a sort of “DNS” for Ethereum addresses. Mine, for example, is
beckermatic.eth. It's intended to be a easy-to-remember address, and a way for projects under Ethereum to have an official, easily-identifiable address (to collect funds, etc).
As it is a non-fungible token, it's not listed in exchanges as other fungible tokens and coins; however, you can buy them in the official site or also on Opensea. IMHO, twitterbot.eth is a good deal, but, as always, DYOR if it makes you more comfortable.