Why is this a big deal? Pakistan is upset that Iran attacked their land without permission. They called Iran's act "illegal" and have stopped Iran's ambassador from coming to Pakistan. This has made everyone worried that things could get worse between the two countries.
Other things to know: Iran says they had to attack because Jaish al-Adli fighters were responsible for bad things happening in Iran. However, Pakistan doesn't believe this and wants Iran to be punished.
What could happen next? It's hard to say exactly what will happen, but everyone is hoping that Pakistan and Iran can talk things out peacefully and avoid fighting. China, a friend of both countries, is asking them to be calm and try to solve this problem without hurting anyone else.