Facing death is common while adventuring, and every now and then, through bad luck or poor planning, death wins. There are two basic stages in this, mechanically: the Wounded condition, which measures how badly you’re beaten up after you drop to 1 Wound, and the Dying condition, which measures how close you are to actually breathing your last.
Each time you are damaged and your Wounds would be reduced to 0 or lower, you instead drop to 1 Wound and gain +1 or more ranks of the Wounded condition. Each rank of the Wounded condition slows you down while you’re at low Stamina, so if you want to keep fighting while you’re Wounded you need to keep your Stamina topped up or else you’ll be fighting with a growing handicap.
You lose the Wounded condition when you once more have 2 or more Wounds, which usually requires either powerful healing magic (such as from a Greater Restoration or a Healing Potion, though such magics have limits to how much they can heal) or taking a Long Rest. Some skilled Field Medics can get someone back into fighting shape with only a Short Rest, but that takes a toll on the body and is viewed as an option of last resort by most.
If you keep getting knocked down below 1 Wound, though, eventually it sticks. When your Wounded condition reaches 4 ranks, you start Dying, causing you to fall Unconscious and begin to bleed out. Each round that you are Dying, you make a Recovery Check at the start of your turn to either start to stabilize or continue to bleed out. Healing and damage applied to Dying creatures helps in this contest with death, so guard your fallen allies well. When your Dying condition reaches 6 ranks, death claims you and you die.
You die immediately if you ever take damage equal to your remaining Stamina and Wounds plus your maximum Wounds. This kind of massive damage usually results in your body being pulped, incinerated, frozen solid, etc., which may make resurrecting you difficult.
Once you die, your allies will need to cast the Revivify ritual on your body to return you to life, or bring as much of your body as they can retrieve to a cleric who knows how to cast one of the improved versions of that ritual. This ritual require expensive material components to cast, however, and the cost goes up exponentially each time you get brought back — try not to die too often, or else eventually it’ll be financially impossible to bring you back.
Note that Revivify has a very short time-horizon for being able to cast it — it’s the fastest way to bring someone back to life, but it generally requires the cleric to be very close-by at the time of death in order for it to be viable.