The upper half of a human body forms what can essentially be called a triangle; the the torso as the base and your arms as the two sides. When you fight somebody, you want your "triangle" to be pointed directly at there center of gravity; which runs like a pole through the center of your opponants torso. I should state that it helps you if you draw this out, for visualizing it. Ideally it should look like two triangles pointing at each other.
Now then, if one triangle is in this ideal positioning, and the other isn't, the one that is facing the other is at a distinct advantage duo to the obvious fact that they are not facing they're opponant; who can now use your arm to block your path to your center with one hand and go to town on your exposed center or distrupt your balance at will.
Triangle Theory is a fairly logical straight forward concept, as I find much of Wing Chun is. A simple but fun excersize for practice of application of it is called "regaining the line"; your partner does one attack to put himself in this advantageous spot, and then you do a block/attack of your own that puts yourself back in control. Hope that answered your question; if not, hope you still found it helpfull.