A blockchain, in general, is a network of computers/nodes that have mutually distributed ledgers, meaning the ledgers are shared with one another, this is to verify transactions and values. Imagine you wrote a value down on a spreadsheet in your computer, then passed it around to everyone, the spreadsheet is your ledger. It is now mutually distributed to other parties. The ledgers are shared so everyone can view this open source and verify transactions, this creates transparency and trust. Imagine this network has 100 computers that communicate to each other. This is a blockchain.
For instance, if you wanted to do a transaction on the blockchain, you would send a Bitcoin to someone. If you were to send 3 Bitcoins/crypto from one person to another, you would want to make sure that the person really does have the 3 Bitcoins to send, or else the person who is getting the 3 Bitcoins might not get them. Typically, when you send the 3 Bitcoins, the network verifies with each other in the network that you do have this money, and as long as 51% agree that you do, then the transaction goes through.
Keep in mind, the Bitcoin network is independent, automated and does not have a bank or middleman overseeing it, other than the network itself as the network is distributed, so there is no single point of management.