What does an aura around a selection icon mean?

If you release the screen with that item selected, the behavior is the same as if you had tapped the Poke ball normally, then tapped the selected item normally. It's a shortcut to get to the Pokémon, Pokedex, Shop, and Items menus.

• Swipe Pokeball left: Open Pokemon menu
• Swipe Pokeball right: Open Items menu
• Swipe Pokeball up: Open Shop menu
• Swipe Pokeball further up: Open Pokedex menu

The icon outline is a visual cue from a swipe gesture, informing you of a selection. Each of the four options also uses a different colored outline. If you use the tap gesture, then the visual cue is unnecessary.