Think of it this way. You wake up one morning on a strange world with no idea where you are or how you got there. The creatures living there are not human and they speak in a language you can't understand. How are you going to figure out where you are, what you should do, what you can eat, what the customs are, what the laws are, what the inhabitants do, etc? You don't have someone at your side to interpret, translate, and explain things (which would be telling) so you will have to figure it out by observing. What can you learn from your observations?
Describe something as you (the character above) would see it, with the limited knowledge you (the character) would have, and try to make it obvious what's going on. That's showing.
I can tell you someone robbed the bank I was at today. Or I can show you by describing: the setting, the sound of the robber charging in with a gun in hand, the way the robber looked with his rubber mask, the palpable fear that hung in the air, the smell of the robber's sweaty body as he rushed by me, the trembling of my body, the action that took place, some of the other people who were in the bank at the time and what their reactions were.
Basically, telling is stating what happens. Showing is painting a picture and letting the reader experience what is happening through various senses by describing action, appearances, smells, sounds, tastes, touch, emotions, etc.
Hope that helps.
Beth