I think what he is trying to say is this: Gaining muscle and losing fat are basically energy opposites:
To gain muscle one needs to be in a caloric SURPLUS and to lose fat one needs to be in a caloric DEFICIT, and thus the basic complication. As a whole its not entirely impossible, as some have used a zig-zag form of dieting (deficit for 5 days, and surplus for 2) with some results (but you see the caloric equation I spoke of in the zig-zag), but muscle growth is compromised. With this said, it is in my opinion, better to do one or the other.
As far as reps are concerned, this is a fairly controversial subject with some basic foundations, and subject to where one is in thier training, and learning what works. Some go heavy (4 to 6), some go medium (6 to 10, 8 to 12, etc), and some go yet a bit higher. I would suggest the medium range repage of some sort as a begining, but EXPERIMENT and find what works. Now, the KEY is PROGRESSIVE OVERLOAD. This takes on MANY FACES, it is NOT only adding weight to the bar, it can be other things. However, in the basic sense, its like this: In the gym you do chest for 3 sets at a weight for say 2 weeks (and reps are going up--progress), once you reach say 12 on the first set, one would add 5lbs to the weight, and repeat (in basic terminology).