Causation (law) is a way of saying who or what caused something to happen. In law, it's important to know who or what caused something because it can determine who is responsible (at fault) for any damages, or harm caused.
For example, if someone has broken into your house, you need to know the cause of the break-in to decide who has done it and who should face the consequences.
Causation can also be used to decide if somebody should get money for damages or injuries caused by another person or company. In this case, it's important to prove that a certain person or company caused the damage or injury. If they did, then they are responsible for it and they need to pay money to fix or compensate for the harm.