A deadzone is a stretch of map that offers the survivor nothing - every pallet in it has been used, and there is no structure worth running to. Being caught in one usually means taking a hit, because there is simply nowhere to go.
Deadzones form naturally as a trial progresses and pallets are spent, which is why chases late in a match tend to be much shorter than chases at the start.
In duels this shapes how a round is paced. Because both players know the map, both know roughly where the ground runs out - and a survivor who lets themselves be steered toward a spent area has effectively already lost the round.