In the NEAR network, validators have access to the transaction pool, and can therefore see them before they execute. This enables validators to analyze transactions for a potential profit and frontrun them with a transaction of their own.
For example, imagine that you make a game where users are paid for solving puzzles. If not handled carefully, a validator could swap a transaction with the valid answer for one of its own and claim the prize. You can read more about this in this blog post.