Characters can become subject to negative conditions such as Hungry, Thirsty, Sickened, or worse. Conditions are added based on the privations or injuries that your character faces, and can usually be removed by correcting whatever affliction brought the condition in the first place.
Most conditions have only one real mechanical consequence; each one applies a –2 penalty to d20 checks (attack rolls, adventuring feats, and saving throws). The penalties stack, so multiple conditions can reduce the mightiest character to a tottering wreck. A given condition only applies once – you cannot be Hungry twice, for example, though you might be Hungry and Starving both.
A few conditions are special, however. These special conditions do not apply the usual penalties to d20 checks and only affect a character as given in their description:
Incapacitated: A character who has been Incapacitated (by being reduced to 0 hit points) recovers only 1 hp per day until fully healed. The character may suffer from additional negative conditions as well – broken ribs, burns, or other injuries – which will apply the usual penalty and remain until all damage is healed.
Blinded: A creature who cannot see automatically fails all vision-based tasks. Attack rolls against a blinded creature have advantage, and the creature’s attack rolls have disadvantage.
Deafened: A creature who cannot hear suffers a –1 penalty to initiative and is more likely to be surprised (opponents gain advantage on surprise Skill checks).
Fatigue & Exhaustion: Fatigue inflicts a –2 penalty on all all combat rolls, saving throws, and adventuring feats until the PC rests for 1 turn (or for a full evening if the fatigue is from lack of sleep or a forced march). A character who is fatigued a second time becomes exhausted instead and suffers disadvantage on all d20 checks.
Stunned: A Stunned character is incapable of movement and can take no actions, but is not helpless. Opponents gain advantage on attacks against a Stunned target, but must still make a roll to hit.
Paralyzed: This condition ‘freezes’ a character, who can do nothing and is helpless. An enemy who spends a full round lining up a shot will automatically hit a Paralyzed foe and reduce him to zero hit points.