Hi! Piper here.
Forgive me if I'm incorrect, but situational irony is when you expect something to happen, but something else happens that's extremely different from what you were expecting. Situational irony is usually comedic.
Example:
Bob is told that it will rain. He brings his umbrella outside. It did not rain, and Bob was left out in the sun. Bob goes to his house to put away his umbrella, but right when he steps outside it starts to rain heavily!