Have you ever heard the saying "One thing leads to another"? When one thing is known for certain to cause another thing, then the first thing can be called causal.
Causal is a variation of the word cause, which should be a clue to its meaning. A cause is what makes something happen: the notebook flew across the room because you threw it, so your throwing it was causal. If a bolt of lightning set a statue on fire, the lightning was causal for the fire. Sometimes, we don't know what caused something, so we don't know what was causal. But if you can figure out "Who did it?" or "What did it?," then you've found what's causal.