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 let attackers within 5 feet land automatic critical hits. 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 a flat −2 per level on all d20 rolls. Check which edition your table uses, since it changes how survivable exhaustion is.