It's a gacha that you can roll as many times as you want. It doesn't cost any gems until you actually pick a Knight and the only Knights that show up are 5* and 6* gacha Knights. When you pull a Knight the red button on the left rerolls, the button in the middle picks that Knight (a pop up comes up that asks you to confirm that you want that Knight), and the blue button on the right backs out of the gacha in case you want to do something else for now.

It can take a bit of rerolling but you're basically guaranteed a 6* Knight for five gems when you're persistent enough. Once you actually pick a Knight the gacha disappears though so make sure that Knight is one you really want.