Hardy bulbs, such as hyacinth, require a number of hours of cold in order to properly set blooms for the spring. If they are planted outdoors in a suitable climate, this happens naturally during the course of the fall and winter. If you want to have them bloom ahead of schedule in pots, you have to simulate this winter rest by "cold treating" them in some way- generally by either potting them up and putting them outside for a few weeks in a protected area or by leaving the bulbs in your refrigerator crisper for an appropriate amount of time. Cold treating does not mean freezing the bulbs, temperatures in the low 40s will generally work fine. Different kinds of hardy bulbs require different lengths of cold treatment, so before attempting this you should probably read up on it a bit first. Btw, the correct term for what you are wanting to do is "forcing" bulbs.