Share-to-Enter is only available for Sliding Scale style Giveaways. Participants get a custom link that they can share by email and social media. If someone enters through that custom link, the sharer and the new entrant both get 1 entry. Here's how that looks:

When someone enters the Giveaway campaign through that link, that person and the referrer will both get one entry to say thanks. It will look like this on your campaign's dashboard:

On the backend, you can turn this feature on or off by heading to Item & Settings, then scrolling down to Share-to-enter.