Blocks are packs or packets of data or information that form the building blocks of the blockchain. It is similar to a page of a ledger that contains records of transactions.
Blocks basically contain the solution to the mathematical puzzle that the miner has solved for that gives him the right to create the current block. It also contains all the transactions in the current block and a reference to the previous block or blocks.
One can think of blocks as a train with each car connected to the last car and each car in the train containing cargo.
The reference or connection to the to the past block or blocks is what makes blockchain so special since it provides a source of continuity from past blocks to the current block and makes older blocks more secure since older blocks are “buried” under the weight of newer blocks.