When something moves, it almost always has a wave pattern to its movement - this can be seen in water most easily, in ripples and ocean waves. A very common type of wave in the universe is electromagnetic waves. Light is an example of an electromagnetic wave, and so is a microwave, for example the cosmic microwave background radiation. We can only see electromagnetic radiation in the visible light spectrum (which is why it's called the visible light spectrum), so microwaves are invisible.