Monster attacks can inflict:

• outright death
• death save failures
• death save rolls
• injury rolls
• wounds
• other special effects (such as a ghoul's paralyze, a crocodile's grapple, a wight's life drain)

Sometimes you can spend HP to avoid these, sometimes you also need to make a save, spend actions etc.

The "wound" in that list is the same as the 14-16 entry on the lingering injury table dmg p 272; that's the mildest entry (since scars don't heal up) but still pretty bad.

Any magical healing heals all open wounds.

Or, someone tending to the wound can heal it non-magically but it takes ten successes (the person tending the wound making DC 15 wisdom checks); the successes don't have to be in a row but you can only try once per day per victim.

The effect of the wound is that your HP max drops by 1 every day!