Normally if involved in an activity during travel a character cannot participate in perception checks to, for example, notice when they are being followed.
As adventurers travel through a dungeon or the
wilderness, they need to remain alert for danger, and
some characters might perform other tasks to help
the group's journey.
Characters who turn their attention to other tasks as the
group travels are not focused on watching for danger.
These characters don't contribute their passive Wisdom
(Perception) scores to the group's chance of noticing
hidden threats. (PHB p.182-183)
A ranger with this ability may be engaged a travel activity (e.g. foraging, navigating, tracking, creating a map, ...) while still being allowed to participate in the party's perception checks.
You gain the following benefits when traveling for an hour or more:
- Even when you are engaged in another activity while traveling (such as foraging, navigating, or tracking), you remain alert to danger.