But-for causation is a big phrase that means “what would have happened if something had not happened?” Let’s say you wanted to bake a cake, but you were missing one ingredient - the butter. If you didn’t have the butter, you can’t bake the cake, right? So the butter is the but-for cause of you being able to bake the cake.
Another example might be if you missed the school bus, but you got to school by asking your mom to drive you. If you hadn’t missed the school bus, your mom wouldn’t have driven you to school, right? So missing the bus is the but-for cause of you getting a ride from your mom.
Basically, but-for causation is trying to figure out what would have happened if a certain event didn’t take place. It helps us understand the cause-and-effect relationship between events.