https://www.gamespot.com/articles/heres-a-surprising-and-cool-uncharted-fact-you-pro/1100-6460243/

10 years later, we're still learning more about Naughty Dog's popular Uncharted franchise.

Animator Jonathan Cooper revealed on Twitter that the red indicator in the UI that shows up when you're under attack is not a health meter, but instead a "luck" meter. When enemies bear down on Drake, his "luck" will eventually run out, and at that point enemy gunfire will kill him. But this explains that Drake isn't just taking bullets left and right and living to tell the story. That is often a criticism of games; that players are bullet sponges.