Imagine if you could draw an imaginary circle around the three notes you're fingering on the 4rth, 5th and 6th strings when you're fingering your E chord on the guitar. Now, if you only played the actual notes your fingers were pressing down(which is only three notes), you would be playing the very three notes that would comprise a complete E Major guitar chord. With just those three notes (which would be E, G# and B) you would have a complete E chord, and no other strings on your guitar are needed. So, now..if you moved that finger shape up just one fret, and still kept that imaginary pencil line encircling the three notes your fingers were pushing down, and you still just play those notes in the finger shape that your original E chord was in (before you moved your shape up one fret being careful not to change the original shape)..you would arrive at F Major..which is one half step above E. Next, if you repeated this process once again..you would arrive at F# (or G flat) Major. Repeat this process once more..and you end up at G Major..and, so forth. Each time your shape moves up by one half step. After G, you would get G#(or A flat), then A, then A# (or B flat)..and there you go. I hope this helps..
