A condition is a temporary status — from a spell, a monster’s attack, or the environment — that changes what a creature can do. Most are bad and you don’t choose them; each lasts until something ends it, whether that’s the end of a turn, a saving throw, a short rest, or a specific cure.
The most crippling are Paralyzed, Stunned, and Unconscious: they make you Incapacitated (no actions or reactions), and attacks against you have advantage. Paralyzed and Unconscious go further — any hit from an attacker within 5 feet is an automatic critical hit. Others are narrower — Prone just costs you movement and hands melee attackers advantage until you stand up.
Exhaustion works in levels. In the 2014 rules it has six levels, each stacking a worse penalty up to death at level 6; the 2024 rules simplify it to −2 per level on every d20 test and −5 feet of speed per level, with death still at level 6. Check which edition your table uses, since it changes how survivable exhaustion is.
Every 5e condition explained
Each condition has its own rules page with the 2014 and 2024 wording, what causes it and how to end it: Blinded, Charmed, Deafened, Exhaustion, Frightened, Grappled, Incapacitated, Invisible, Paralyzed, Petrified, Poisoned, Prone, Restrained, Stunned and Unconscious.
Related combat rules: cover (half, three-quarters and total), concentration and death saving throws. Browse them all in the 5e conditions and rules hub.