To use tokens and NFTs in decentralized applications such as Uniswap or OpenSea you have to grant the application permission to spend tokens on your behalf - known as an allowance. Allowances are an important part of using these apps, but can be dangerous if left unchecked.
To read about token allowances in more depth, make sure to read the article Unlimited ERC20 allowances considered harmful.
To revoke token allowances you can use revoke.cash.