ELI5: Explain Like I'm 5

Impedance of free space

Impedance of free space is like how much something (like sound or light) can travel through empty space. It is measured in something called ohms which tells us how much something has to push against to travel through a space. The amount of ohms that empty space has is a big number, 377 ohms. That means it is really hard for things like sound and light to travel through empty space.