Some smoothing keyboard shortcut

Hey guys, I’m new to blender. I actually discovered some shortcut key that seemed to refine an object model. I think there were 2 toggle keys. I remember I tried it on the blender monkey head. It actually toggles low poly to hi poly as I observed the monkey head get smoother. But I can’t remember what the keys were and where I can find a tutorial about it. Can you help me out? Thanks.

You’re probably thinking about the hotkey for enabling the Subdivision Surface modifier. Try Ctrl+1