A wallet drainer is a tool built by scammers with one goal: to empty your wallet the moment you connect it and approve something.
These usually live on fake websites dressed up as a real project, an airdrop claim, or a giveaway. You are prompted to connect your wallet and sign a transaction. What looks like a harmless connect or claim is actually a request to approve moving your tokens. Approve it, and the drainer sweeps everything it can in a single transaction.
The trick is that the malicious request is disguised. It might be buried in a signature prompt that beginners approve without reading, because they assume connecting a wallet is always safe.
The protection is to slow down at the approval step. Read what you are actually signing. Be deeply suspicious of any site pushing urgency, free tokens, or a surprise airdrop that requires you to connect and approve.