A condition is a long-term effect applied to a character in a variety of ways. Commonly, an action or other effect will state that a character gains a particular condition, such as: "the character is now afraid". In the case of weakened, it is applied as a result of suffering a certain threshold of damage.
When a character gains a condition, it should be recorded on their character sheet. Narrators should also record NPCs who have gained conditions. When a condition is removed, this should also be recorded. When a character has a condition, its effects apply to that character until removed.
Some conditions stack - this is described in the condition itself; stacking conditions can be applied multiple times to the same character, their effect adding on to itself. Other conditions do not stack (the default unless otherwise stated) - in this case, if the character already has the condition, they do not gain it again, and the effect is ignored.
When a condition is removed from a character, only one copy is removed in the case of stacking conditions.
Some conditions remove themselves automatically at a set time (typically at the end of the encounter). Others persist until some other effect explicitly removes them.

List of Conditions