Every singer needs good breath control, right? But what IS breath control? And what does it mean to sing from the diaphragm? To begin with, you can NOT literally sing from the diaphragm because it's an involuntary muscle that you can't contract consciously. However, the diaphragm is analogous to holding a full balloon in between your fingers and controlling the stream of air coming out in a steady fashion. In other words, you resist the free flow of air out of your lungs using various muscles such as the intracostal muscles during exhalation and this activates the diaphragm as a result, but I repeat – you do NOT directly have control over this muscle in the same way that you don't have direct control over your heart (although you can influence it via breathing, meditation, etc.)
Anyhow, it doesn't necessarily matter how you describe it, the goal is to learn how to create an even flow of air as you sing, so that you can maintain accurate pitch, legato (smoothness of the melodic line) and beautiful tone. When you inhale, you'll feel your ribs moving up and out. This area is called the “carriage”.
As you begin to sing, you'll feel that the ribs have a natural tendency to collapse. You should resist this tendency with as little force as necessary to keep the carriage in a slightly elevated position. This causes the diaphragm to contract and direct the column of air steadily through the vocal folds. You don't want to overdo this. In fact, let me share with you the most important thing I've learned in my life about singing. It took me YEARS to figure this thing out, and here it is: nothing in singing is black and white. When a teacher tells you to “keep the carriage elevated”, to YOU that might mean to use 50% muscular contraction, but for another singer with a different kind of physiology and mentality, it might mean 90%, which would result in way too much muscle tension. In other words, you MUST become very in tune with your body and learn to listen to it so that you can find the optimal amount of any given instruction.
And “optimal” in this case means whatever amount of effort causes the greatest positive effect with the least muscular tension, but be aware that some muscle tension is always involved, and anybody telling you to completely relax doesn't know basic anatomy and physiology. Make no mistake about it, singing is a sport.
If you're just starting out, you probably don't feel this to be true YET, but after a while, you'll definitely appreciate how athletic singers truly are. This doesn't mean that singing is a balls to wall event where you go 100% all the time; quite the opposite. You're learning to control your body efficiently to produce a desired sound, just as a runner learns to control his or her breathing, muscular contraction and a million other unseen factors to achieve a fast speed.
It's not necessary to be in good shape to be a great singer. Look at Pavarotti; he was rather chubby. However, that fact belies how athletic and muscular he was underneath the fat. Learning to employ the muscles surrounding the diaphragm is as vital as keeping a neutral larynx. Keep practicing your singing and you WILL secure ever-improving breath control.