What it means
Email verification checks whether an address can receive mail before you send to it. A verifier confirms the syntax, looks up the domain's mail servers and asks the server whether the mailbox exists, without sending a message. The result is usually valid, invalid, catch-all or unknown.
Sending to invalid addresses causes bounces. Bounces damage the sender reputation that every other email from that domain depends on, so verification protects the whole campaign. It is also a point-in-time check: an address that passes today can fail in a few months, because people leave jobs and mailboxes are shut down.
How we use it
Each address is verified before it enters a sequence, so bounces never reach a client's sending domains. Older lists are re-verified before reuse, because people change jobs and an address that was valid last quarter may bounce today.
The Email Verification Waterfall skill does both halves of the job. It verifies what you have, and when an address fails, it finds a replacement and verifies that too.