Key checks are regular check-ins for each of the keys that are part of your multisig vault or loan. We request that at least once every 90 days you login into your account and sign a transaction verifying that you still own the key.
If you skip this 90-day check, you will not be able to create a transaction or create a new multisig address from your Unchained Capital account with the degraded key until you verify that you still have the key and can sign. Your vault is still fully protected and you can spend your funds externally with Caravan or another external multisig wallet, but you will need to complete the key check before you are able to transfer bitcoin from within your vault or loan. Here's a short video guiding you through the process.