A condition is a temporary state a creature can be put into, and the rules use them as shorthand: rather than spelling out what being knocked flat does every time it happens, a rule hands you the Prone condition and the condition says the rest. There are fifteen of them, each with its own entry in the glossary.
Whatever applies a condition also says how it ends — when the spell stops, when the creature saves, at the end of its next turn, on a successful check. A condition with nothing to end it is a rule someone has misread.
This is the part that surprises people. You either have a condition or you do not. A creature already Frightened that is frightened again by something else is not more frightened; it is frightened, with two things that might end it.
Exhaustion is the one exception, and it works by levels. Each new dose adds a level, and each level costs you: every D20 Test you make is reduced by twice your level, and your Speed drops by 5 feet per level. At level 6 you die.
It comes off slowly — one level per Long Rest — which is what makes it the game's clock on attrition rather than a status to shrug off.
Conditions are not independent of each other, and this is the detail worth knowing before it comes up mid-fight.
Incapacitated is the one that stops you playing: no action, no Bonus Action, no Reaction, no speech, and any Concentration you were holding is broken. It is rarely applied directly. Instead it arrives as a passenger: Paralyzed, Petrified, Stunned and Unconscious each impose it as part of what they do. So the answer to "can I still take a Reaction?" is usually found one condition further down than the one you were given.
Unconscious carries a second passenger, and an odd one: it also gives you Prone, and Prone stays behind when the Unconscious condition ends. You wake up on the floor.
Everything above is about how conditions behave as a group. What any single one actually does is in its own glossary entry, and those are the definitive text — Blinded, Charmed, Deafened, Grappled, Invisible, Poisoned and Restrained are the rest of the fifteen. Most of them come down to Advantage and Disadvantage pointed in one direction or the other; the ones that do more than that are the four that hand out Incapacitated.