Blockchain is a technology (distributed database) with the main purpose of being immutable (no one can change the data) and trustless (no individual entity has enough sway to require you to trust them). 

It is a chain of “blocks”, where each block contains information. This information can be anything, and in the case of bitcoin it is transactions. Blocks have set amounts of information they contain, then they link to the next block with a hash (realistically unbreakable encryption). This means you can look back through the chain and make sure each block is valid by their hashed links.

Any computer can be a node in the network, meaning they are contributing to the blockchain. Each node computer stores a copy of the chain and compares it with everyone else’s. This means that if someone tries to add false information to the chain it can be discarded by comparing it to the many real versions being broadcasted on the network. The more individuals participating in the network, the more secure it is.