A blockchain is a ledger that's distributed over thousands of computers connected to the internet. All these computers hold the same ledger, and software that updates the ledger as soon as any of the individual computers makes a transaction, and all computers constantly check each-others copy of that ledger to assure everyone has the exact same copy at any given time; this is the basic functioning of all cryptocurrencies. Instead of just data you can also put applications on that distributed ledger (or cryptocurrency). STEEM is one of those and on it runs Steemit, DLive, DTube etc. EOS is another one that's supposed to be even faster so it can run more, and more complicated distributed applications, or Dapps. Imagine a game on such a distributed system: if one player earns something, that information is sent to all other player's computer... So we now have network-applications and currencies that don't need a central server or central storage space or permission to do thiongs from a central authority. Decentralized internet is within reach...
Wow... difficult question you ask there @meno... ;-) I think just taking the dive and start using the stuff is the best way to learn. Else there's always Andreas Antonopolis to explain things... ;-)
Oh man... Andreas is the best, I've learned tons from that guy... but even his explanations go over the head of many people.