A blockchain is like a chain of blocks. Each block is linked to the one before it, except for the first block, which is special. Each block has a code called a hash, which is like a fingerprint. If someone changes something in an earlier block, the code for that block will change. But the code for all the blocks after it will stay the same. This means that the chain of blocks will no longer match, and will be broken. Changing a block is very hard to do, and it takes a lot of time and computer power. To change many blocks, a person would have to control more than half of all the copies of the chain. This is very difficult to do and would cost a lot of money.