Proof-of-Work is the most common type of consensus. It's the one Bitcoin uses.
In Proof-of-Work, users called miners validate and collect transactions into blocks and seal them in a complex cryptographical way.
Proof-of-Work requires a lot of computational power and electricity.

There are other less popular consensus types, such as Proof-of-Stake. PoS systems choose special users to validate transactions. These users are selected based on the amount of coins they deposit. So if they validate bad transactions, they will lose all their deposited coins.

If you want to learn more, read this article: https://finrazor.com/news/blockchain-consensus-mechanisms