Something happens simultaneously when two or more things happen at the same time.
You've probably experienced it before - doing multiple tasks at once, like brushing your teeth and washing your face. That's simultaneous! When you're multitasking, like cooking dinner while chatting with a friend on the phone, that's simultaneous too. It's also used to describe events happening together in the world outside - for example, "The fireworks started simultaneously with the music." Or think about two friends meeting at the same time and starting their hike: they're doing it simultaneously.
Occurring at the same time.
