What is Email verification?
Checking whether an address exists — by asking its mail server — before you send to it, to keep bounces under 2%.
Verification asks the recipient's mail server whether an address is deliverable without actually sending a message. Verdicts are typically valid, invalid, catch-all (the domain accepts anything, so it can't be confirmed) or unknown (no clear answer, often greylisting). Verifying before sending is the cheapest deliverability improvement there is; sending unverified purchased lists is the most common way domains get burned.