It's actually a pretty basic theory concerning capoing. But I can create a bit of a headspin. When you put a capo on the guitar, the notes that you're capoing now become your open strings. So essentially, when you put a capo on the guitar it's like you're moving the nut of the guitar. For example, if you were to put a capo across the fretboard on the 2nd fret and play a regular open G chord (based on the 3 fret from the capo) you would actually be playing an open A. You would use the same voicing and be in the same position as an open G, but you would hear an A chord.
Basically you're doing the same thing when playing barre chords.