Short and consice..

Okay

1. What is blockchain?
   A blockchain is effectively a database But:
   a) Once a data is stored in the database, it can never be modified or deleted for ever, Record on the blockchain is permanent for eternity as long as it's being maintained
   b) no single individual or entity maintains the blockchain, several entities do. And everyone of them as a copy of the blockchain that is constantly being checked and reviewed by each other.

2. Why is Steem called a Blockchain?
   Because Steem is designed as a blockchain with some newer technologies implemented in it as compared to the Bitcoin Blockchain that aims to disrupt the Content Web. Every activity performed on the Steem Blockchain gets recorded and can never be deleted as long as there are people (whom we call witness) maintaining it.

2.a) But why am I able to edit posts and comments, pull back upvotes and flags then if its immutable?
Because technically it doesnt get deleted on the blockchain, when you edit a post what happens is the newer data gets registered on the blockchain that links to the original data, so although your original data is still stored in the Steem Blockchain, Steemit.com the website is smart enough to read the newer data and shows the "edition" as the edited post. Meaning even if you edit a post, no deletion was actually done on the blockchain, so it's still immutable.

PS... This is as short as i can explain to get the points across, I am not a programmer nor a blockchain expert but because how new this technology is, explaining them can still get quite nerdy.. It's like explaining the Internet but talking about TCP/IP, which is where decentralized Blockchain is right now currently.