Through the entire MCU with exception of Captain Marvel, we see Director of Shield, Nick Fury wearing an eye patch. When he revealed his eye in Captain America – Civil War, it was shown that it is infected with something. This was explained in Captain Marvel. For the first time we see Nick Furry without his eye patch.
He lost his eye because of a cat friend named Goose. Goose is not actually from earth. Goose is a Flerken and the former pet of Mar-Vell. Flarkens are extremely dangerous creatures that resemble cats. They have pocket dimensions inside their bodies that can swallow up items for safe keeping.
Goose was one of the few things Nick Fury licked. In an encountered where Nick was overly petting him, he scratched Nick’s eye brows with his claws. At first it seems like a normal thing a cat would. The wound got infected and spread to his eyes causing permanent lose of sight in that eye.
In Civil War, Nick mentioned that Goose was the last thing he trusted and it took out an eye. We are not sure if Goose might appear in Avengers: Endgame but would be interesting if it does.