In the MCU, Uru is the material Thor's axe Stormbreaker, (and possibly Mjolnir IIRC), are made of. Is there any indication what this metal is?
Uru is not a metal per se, it's a metallic ore. Its appearance is stone-like with metallic properties. The ore is primarily sought after for its ability to store energy, particularly magical ones.
It is found and mined only in the Nidavellir realm. It is one of the most prized exports of the dwarven miners that reside there.
You are correct about it being the material used to make both Stormbreaker and Mjolnir. Apparently, it was also used to construct Odin's spear - Gungnir, which he can be seen holding in previous Thor movies. It is said to occur in two variations in terms of appearance.
The appearance of Uru metal ore is in two forms, one as "metallic gold" (Gungnir, Stormbreaker) and the other as having a luster that has been described as "badly wrought iron" (Mjolnir). When Uru is enchanted, it becomes much more durable.
From MCU wikia,
Uru was a godly metal forged in the furnaces of Nidavellir.
Mjolnir, Stormbreaker and also the Infinity Gauntlet were forged using Uru.