The hook
A site claims your wallet needs "validation" or a small gas fee to release a reward or fix an issue. The payment or signature is the actual attack.
What really happens
Either you send a fee for nothing, or the "validation" is a signature that approves access to your tokens. There is no reward.
How to protect yourself
No legitimate process requires paying a fee to "unlock" your own funds or validating your wallet by signing on a random site. Treat any such request as a scam and leave.