List hygiene covers verifying addresses at signup, removing hard bounces immediately, suppressing long-dormant or non-engaging recipients, and honoring unsubscribes and complaints without delay. It's a continuous practice, not a one-time cleanup.
Poor list hygiene is the most common root cause behind spam-trap hits, elevated bounce rates, and slow reputation decay; almost every deliverability problem traces back to it eventually.
Re-engagement campaigns (asking dormant recipients to confirm continued interest before excluding them) are a common middle ground between mailing an entire stale list indefinitely and suppressing anyone who hasn't opened recently.