A contact created or updated by email, added to the list you choose and tagged so your automations can fire off the tag. Mapped answers are written to your account's real custom fields.
Alongside the answers themselves, each lead carries the campaign and ad it came from, the Facebook Page name and the time it was submitted, so you can see which ad is actually growing the list, not just that a subscriber appeared. Every lead is also scored before delivery, with real checks on the email address, so an obviously fake one never gets the chance to bounce and hurt your sender reputation.