A laceration is one of the Afflictions that can affect the player in The Long Dark. It is usually caused from getting bitten or mauled by Wolves or a Bear, but it can also be obtained from a heavy fall.
While the player has a laceration, they will steadily lose Condition until the affliction is treated, or the player dies. They will also leave a blood trail, which wolves can follow.
To treat the condition, the player needs to use a Bandage or an Old Man's Beard Wound Dressing. The dressing has the added benefit of also treating infection or infection risk, but if the player has neither of these conditions, using the dressing would be a waste.