Insulin is a hormone secreted by the pancreas in response to detected increases in blood sugar - insulin controls glucose homeostasis (i.e., keeps blood glucose levels constant) by stimulating the uptake of glucose into skeletal muscle and, to a lesser extent, into liver (e.g., in the case of fructose) and adipose tissue.
In laymen's terms - it is essentially responsible for storing away the latent energy in the calories you consume. Insulin drives glucose into your cells - it feeds your muscles as it were.
Foods that are high on the glycemic index (e.g., dextrose, or other "simple" carbs) stimulate a strong spike in insulin - "spike" just means that insulin levels rise and fall rapidly (good for a quick burst of energy but not for sustained endurance). Foods lower on the glycemic index (e.g., so-called complex carbs) stimulate a much slower insulin response allowing for a sustained release of energy.
Generally speaking - bodybuilders desire an insulin spike only after a workout (they generally combine simple sugars with protein) because they want the insulin surge to "feed" their muscles as quickly as possible (trying to use laymen's terms here!).
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