A token approval is a permission slip. When you use a decentralized app, it often needs your permission to move a certain token on your behalf, so instead of approving every single action, you grant an approval once.
The risk is in the fine print. Many apps request an unlimited approval, meaning the contract can move any amount of that token, at any time in the future. With a trustworthy app that is usually fine. But if the contract is malicious, or is later exploited, that standing permission can be used to drain that token from your wallet long after you forgot the approval existed.
The safer habit is to approve only the amount you actually need, when a wallet offers that option. It also helps to periodically review and revoke old approvals you no longer use, using a reputable approval-checker tool, so no forgotten permission is left open.