Reduced glucose tolerance is another phrase for insulin resistance.
Under normal conditions, when "x" grams of carbohydrates are ingested, the body secretes "y" amount of insulin to remove it from the blood stream and drive it into insulin-responsive tissues. Under "reduced glucose tolerance" situations, when the same "x" grams of carbohydrates are ingested, "y" amount of insulin is not sufficient to effectively remove it from the blood, so the body produces 1.5 "y", 2 "y", 2.5 "y", etc. Also, basal (baseline) insulin levels are usually elevated under these conditions.
Birth control pills are common causes of glucose intolerance, due to the progestagen in the preparation. Progestagens increase lipolysis and free fatty acid levels in the blood, which interferes with glucose metabolism.