You can visualise a blockchain like a huge spreadsheet, in which all transactions of that system are stored sequentially and chronologically.
In essence, it is the equivalent of a ledger, a register in which everything that happens in the network of a specific blockchain is recorded. By looking into it you can see all the movements happening on that specific blockchain.
All those movements and transactions are stored in concatenated blocks, hence the name 'block-chain'. Each block contains a set of transactions and is linked to the previous one.
When two people want to make a trade, the data is sent to all of the computers on the blockchain network, where it gets validated. Once that happens, it's added to a block. This block is then giving it a virtual fingerprint, so it's always unique. And finally, it gets added to the blockchain itself and sent to all computers on the network—completing the trade in a transparent and decentralised way.