A pool becomes inactive when a candidate/validator elects to stop participating in the consensus process. This is different from a ban, where a malicious validator is immediately removed from consensus.
If you see a current validator’s pool status as inactive, the validator will exit the validator set once the current staking epoch is complete. Prior to the end of the epoch, the validator will continue with their responsibilities, and any delegators will receive rewards once the staking epoch is finished. Rewards may be diminished if the validator’s participation was limited during that epoch.
Inactive pools are not considered for the next validator set, and will not become validators again unless the pool is reactivated by the validator. Delegators who remain in an inactive pool for the next epoch will not receive any staking rewards for that epoch.