Kakashi arrives just in time to save Obito from a hidden Stone Ninja, losing his left eye in the process. Inspired by Kakashi's sacrifice, Obito awakens his Sharingan for the first time, allowing him to see through the enemy's camouflage and kill the attacker.
You can find the English dubbed version of this episode on major streaming platforms: (Availability varies by region) Hulu (Common for US viewers) Netflix (Select regions) Naruto Shippuuden (Dub) Episode 120
After rescuing Rin from a cave, the enemy triggers a "Rock Lodging Destruction" jutsu. To save Kakashi from a falling boulder, Obito pushes him out of the way and is pinned under the massive rock, crushing the entire right side of his body. Kakashi arrives just in time to save Obito
Knowing he is dying, Obito offers his remaining left Sharingan to Kakashi as a gift for becoming a Jōnin. Rin, a medical ninja, performs the transplant immediately. To save Kakashi from a falling boulder, Obito
It clarifies why a non-Uchiha like Kakashi possesses the Doujutsu.