Before is a word we use to talk about things that happen or exist in time that has passed. It's like when you say something happened 'before' now, it means it happened in the past.
For example, if you say 'I brushed my teeth before going to bed,' it means you did the teeth brushing 'before' you went to bed, which happened in the past.
Imagine you have a plate of cookies. If you eat some of the cookies before dinner, it means you eat them before you have dinner, which happens later. You can't eat before you have cookies, right?
Another example is if you go to bed before your bedtime. Your bedtime comes after you go to bed, so going to bed before bedtime means it's a time in the past.
Before helps us understand the order in which things happen, and it's a way to talk about the past compared to now or something else in the future. It's like a signpost that points to something that already happened or existed.