Soundstage is when you close your eyes and say the guy playing guitar is there, the drums are a little to his left and the bass player is even more left than the drums. You no longer hear your individual speakers but just sound as if the band is in the room with you.

There's lot's out there on speaker placement for stereo. Here's one from my current favorite speaker provider:
Stereo Speaker Placement

You'll know when you get it right.