Uru is an exotic resource you can collect in Marvel’s Avengers to upgrade your artifacts. Artifacts modify the powers and effects of your hero’s abilities and physical attacks during combat. You have two types of artifacts: minor and major. Minor artifacts only affect the hero wielding it. A major artifact is shared across all of your heroes.
You can receive Uru by finding caches and boxes around the map while you play the game. The boxes have distinct glowing colors, from blue, purple, and orange shades around them. The different colors hint to the quality of resources inside them and the rarity of items you can receive. Some of them are tucked away on the map, and others are highlighted for you by Jarvus. Most of the time, you can see Uru pop up as one of the resources inside.
Alternatively, you can earn them by completing Elite Hives. An Elite Hive is an AIMs headquarter hidden in the world that you need to uncover and conquer. You and your team will face increasingly powerful defenses attempting to prevent you from doing so, and depending on the difficulty, will offer more rewards.
When you’re ready to upgrade your artifact, open the gear tab while controlling your chosen hero and pick the artifact you want to improve. It should show many Uru you need to upgrade it, and what you receive in its modified form.