Gifters

I am new to chess and chess.com. So there is a leaderboard of "Top Gifters". Who are the gifters and what is this leaderboard based on?

These are the generous souls who have taken advantage of the ease with which memberships can be given away.  Click on any member's name or even just run your cursor up to it and various icons appear; the one in blue that looks like a diamond with a small + sign next to it allows you to gift a membership to that person.  Can be for any reason, but it's usually a recognition that the recipient is a serious student of the game and would benefit from the additional resources chess.com offers to premium members.  We do see some folks, usually kids, begging for free membership in the forums; I honestly don't know how that usually works out.

To add to what @notmtwain said, it's for members that pay money for other members to get memberships through that process 

https://www.chess.com/members/gifters

You're number 15 on it