yosler:
If don't really understand what a 'phase' actually is, that would help.
Here's one description:
In essence, for (say) a sine wave, the phase describes how "far along" the wave you are at a particular moment. Electricity distributions systems usually use 3-phase power, the idea being that the 3 phases will roughly cancel out when added together, reducing the size of the "ground return" wire that is required.