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. You can use antiseptic or an Old Man's Beard Wound Dressing to treat infection risk, which usually comes with the laceration, but if the player doesn't have that, using the dressing or antiseptic would be a waste.