A Blockchain is a distributed network of containers with data, a hash, and a pointer to the previous hash.  A hash is a text that consists of an input of all the parts of the block, and it will generate the same hash every time the input is the same.  So if the block has been tampered with the hash will be different. On top of this there's a global record of the transaction that's distributed to everyone in the ecosystem.  Making it almost impossible to to tamper with because if a hash has been tampered with there needs to be a consensus of the change before it is deemed valid.  A consensus consists of 50% of the users.

If the hash, data, or anything of block 1 changes, the block 2 becomes invalid because block 1 will have a new hash and block 2 will be pointing to the wrong previous hash, making everything after it invalid, therefore it is in everybody's interest that changes are made very difficult because it would greatly impact the integrity of the blockchain.  There are many checks and balances that must be met before a new block is accepted in the blockchain. Obviously there needs to be something that will correct a bad new block chain, else if a new block chain is bad then nobody can add a new chain to the blockchain anymore.