What is Wallet drainer?

Malicious code or a signed transaction that moves all approved assets out of a wallet.

Drainers usually ask you to sign an approval or a 'claim' transaction. The signature is valid, and the contract uses it to transfer what you allowed.

The defence is reading what a signature actually authorises. If a site cannot explain why it needs an unlimited approval, do not sign it.

PhishingSecurity
Fake sites and messages that imitate a real service to capture keys or approvals.
Token approvalSecurity
Permission you grant a contract to move a token of yours, sometimes without a limit.
Seed phraseSecurity
Twelve or 24 words that regenerate every key in a wallet; never type it into a website.

See it on a coin page