The blockchain or chain of blocks, called in technical jargon, is a huge distributed database that contains information using alphanumeric codes.
Suppose there are 3 people (bob, Peter and Mark), all three have to track their money movements and make a ledger.
On the ledger they report all the operations done every day, entering the amount the date and to whom the money went.
Now bob during the night, without everyone’s knowledge, tampers with the operations on the book by inserting other amounts, modifying all the operations.
The other two if they realize the day after each operation, insert a hash code made up of numbers and letters, where this code contains the data of each operation.
Each hash code is linked to the previous code, so if a person should change the code then he must change all the previous codes, in all the notebooks.
In addition, each ledger that contains the codes is distributed to all three, where everyone reports the code in their books at the same time.
Bob uses more than two days to be able to change the codes but without success.
This is because it would take a long time to change the codes from the beginning, especially if you have a lot of operations it will become impossible.
In addition, I redeem the distributed notebooks, you will have additional security when you will have to tamper with all the ledgers simultaneously.
So the Blockchain is this, it uses alphanumeric hash codes the sequences inserted inside a block, all linked together that report all the operations performed, in distributed registers.
Everyone who is part of the network has a copy of the register with the codes and can validate the blocks together