What is the difference between a main and candidate validator?

A main validator is an active node responsible for validating transactions, producing blocks, and securing the network.The Validators with the most amount of staked CHZ are main validators, currently the top 13 validators, however this is subject to change.

A candidate validator is also an active node that meets the requirements to become a main validator but is not yet actively participating in block production. Candidate validators are usually waiting for an opportunity to be promoted, which can happen if the number of main validators is increased, if the candidate gets more CHZ staked and enters the top 13 or when one of the main validators go to jail.

Essentially, main validators are actively securing the blockchain, while candidate validators are in a standby role, ready to step in when needed.