Decay is a player survival mechanic.
The air of the Betweenlands slowly decays the player. The decay bar can be seen above the hunger bar in the dimension, appearing as this:
It will decrease at a relatively constant rate. As decay points drop, the player's maximum health is reduced, up to a maximum health value of only 6 health, and they are afflicted with increasingly more potent amounts of the Slowness effect, up to level III. Their skin will also receive a green overlay to indicate they are physically decaying. Like hunger, decay has exhaustion and saturation values that decrease under similar conditions.
Decay can be reversed by consuming Balls of Sap, Weeping Blue Petals, or Sap Jello, or by drinking an Elixir of Ripening. Different consumables restore different amounts of decay, and in some cases apply a special effect that makes the decay regenerate over time.