This is more of a Madcow answer, but I'll do my best. The 5X5 theory came from Bill Starr. The 5X5 is a strength routine. The thoughts behind it are that 5 sets 5 reps, with a max weight are one of the best target areas for size/strength increases; doing only compound movement exercises - bench press, bent rowing, squats, deadlifts, pullups, standing military press, etc. If you can do more sets and/or more reps, you should increase the weight so that you basically can only do 5X5. See Madcow's link for the 5X5. If you're new to weight lifting, I'd suggest that you learn all of the basic lifts before starting any routine like this.