An “as easy as possible” mode to let new players enjoy exploring as they follow along with the narrative.
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You want story and adventure, but aren't interested in the challenge and combat. Camera and aim assistance settings are enabled by default.
Uncharted 4's Explorer Description.

Explorer is a new difficulty that was introduced in Uncharted: The Nathan Drake Collection and is a difficulty in Uncharted 4: A Thief's End

Explorer is the easiest difficulty, surpassing the Very Easy difficulty, in contrast to Brutal, the other new difficulty introduced in The Nathan Drake Collection, which is the hardest difficulty.

This difficulty is for players who want to play the game simply to enjoy the story and breeze through the games without having a real challenge, allowing for a more narrative driven playthrough. The notable changes are that Nathan's health is significantly increased, allowing for him to sustain a hefty amount of fire from enemies, as well as having spontaneous regeneration. The enemies' AI is relatively primitive, and is not as intelligent as it is compared to other difficulty modes. Also, that the enemies' health are very low, and can be killed in a few bullets and/or instantly with melee.

In Uncharted 4: A Thief's End, upon selecting explorer difficulty, 3 settings from the Accessibility section of the Options menu are enabled by default: Camera Assist, Vehicle Assist, and Lock-on Aim, though they can be disabled optionally.

In Uncharted: Drake's Fortune, it is a new difficulty introduced above Easy, in Uncharted 2: Among Thieves and Uncharted 3: Drake's Deception, it replaces the Very Easy difficulty, and in Uncharted 4: A Thief's End, is the difficulty above Light difficulty.