Omnium Stars are a collectable item registered in your Collectables window (Alt + L).
There are currently only 6 Omnium Stars in total on EU/NA
The collection rewards include skill points, the legendary Wealth skill rune and stat potions.
Once you have collected a Omnium Star, you must go to Nia Village in Punika and speak to “Albion” to claim the Collection Rewards.
How to obtain Omnium Stars