[PeterisP]

Mining is a "proof of work" event - in essence, you generate a random value so that a cryptographic hash total block would end with a particular number of zeroes. Such a block + hash is trivial to verify but very time/effort consuming to create, thus proving that you spent a large amount of (otherwise useless) work to "mine" that block.

Having a large amount of work required is what prevents a malicious actor from tampering with the chain, since doing so requires doing as much work as all the other miners together.