Think of your body as an old steam train engine. The wood/coal is your food.
If you don't stoke the fire in the boiler often enough, steam goes down, and the train goes slower...eventually stopping when the fire's almost out.
If you throw too much wood on the fire, it smothers it, and you lose steam.
But if you add just the right amount of wood, at the right incremental time period, the fire stays at peak temperature.
Our bodies can only process a certain number of calories at any meal...you eat too much, you get fat and feel sluggish. Eat too little and you're sluggish, and not gaining muscle...and potentially stunting fat loss.
The real, heavy-weight bodybuilders eat 6-8 meals a day. The post workout meal is always a protein/carb shake, high in simple carbs. Aim for 30g protein and 50-60g of carbs, along with any supplements.
The other 4-5 meals in the day should all contain protein, and except for the last meal, a good number of complex carbs. I personally think the last meal of the day, prior to bedtime should be a slow digesting protein like cottage cheese, limit the carbs, go for some extra fiber, and flaxseed oil to further slow digestion. (some bodybuilders actually get up at 2 am and down more protein...not needed unless you're actually competing).
Diet is usually the problem area for anyone who's weight training and not seeing gains in muscle mass or strength. If you want to grow, you need steady nutrient intake, and a surplus of calories (in particular protien and carbs) so you can not only repair damaged tissue, but rebuild it stronger than before.
If you're in a bulking phase, you can cheat on the diet a little more, since it helps get you more carbs and overall calories. But in reality, bodybuilders tend to eat chicken/turkey breasts, fish, and not a lot of red meat. Most of their carbs are from oatmeal and brown rice..and sometimes pasta (wholegrain). Tons of fruits and veggies, and protein powders to add extra protien to the diet.