When we say a sentence, we sometimes have to use a "time" word or phrase to explain when something happened. In Latin, this is called a "temporal clause." It's like a little side sentence inside the bigger sentence that tells us when something happened.
For example, let's say we want to say "I ate breakfast before I went to school." In Latin, we might say "Ego prandium comedi antequam ad scholam ivi." "Antequam" means "before," and it's the temporal clause that tells us when I ate breakfast - before I went to school.
Using temporal clauses can be really helpful when we want to be really specific about when something happened in a sentence.