Normally, after a tooth is extracted, a blood clot forms over the hole, creating a temporary shield. Dry socket can happen when the blood clot becomes dislodged and falls out, exposing bone and nerves and causing significant pain.
In an effort to prevent this from happening, your dentist will give you a list of instructions, such as not drinking out of a straw and avoiding certain foods, to help protect the clot and keep it in place.