A condition is a requirement that must be met for something else to happen. For example: if you turn off the tap while brushing your teeth, you will save water; if you leave the lights on unnecessarily in your room, you will waste resources; if you don't eat properly, you will get sick.

A condition is something that if fulfilled produces an effect. If it is not met, it produces the opposite. This is key to giving the right orders to our objects. If you don't make the conditions of your orders clear... they won't obey you.

See it more clearly with Jordi's explanation.