em is a shortening of electromagnetic.

an em wave is a form of tranverse wave (like water waves)... this means that the wave propagates perpendicular to the disturbance. for example when you watch water waves coming towards on the beach the movement of the wave is vertical but the wave is travelling horizontal toward you.

the way that this is normally thought about is a line of particles (blocks of water) moving up and down out of sync (phase) with each other. if you watch an individual particle you'll see it moving up and down as the wave passes. as in the gif: as the sine wave goes from left to right the green dot goes up and down.

currently the model of the world most commonly used says that all things are both 'waves' and 'particles' at the 'same' time... or things called 'waveicles' or debroglie waves... this basically means that under some conditions (impossed by the observer) things behave in a 'wavelike' way and in other circumstances (again impossed) they behave in a 'particlelike' way. for example 'a' human can be thought of as a very high frequency em wave, although this is not usually done because it does not give useful results... a microwave can be thought of as a very low momentum particle.. this is again not usually done due to it's lack of usefulness as a model.

for most stealth applications we use the wavelike properties of matter... these are various and include: reflection, diffraction, refraction, polarisation and something called the photoelectric effect.