Reorganizing your cards allows you to determine which cards will be gaining mana from gems.

The first cards in your hand will gain mana until it's full, followed by the 2nd, 3rd, etc…

To change the order of your cards, tap and hold a card until it lifts out of your card pane, and drag it to the position you want it to be in.

Cards can also be exiled/removed by dragging them into the trash icon on the right side of the screen.