In simple terms, dry needling involves the needling of a muscle's trigger point or muscles that are causing discomfort. The trigger points are areas of knotted or hard muscle. The needle helps remove the knot and relieve the muscle pain and spasm. It is called "dry" because there is not any kind of medication being injected into the body.
Dry needling is not the same thing as acupuncture. The only thing they have in common is they both use thin, stainless steel myofilament needles. Acupuncture is based on Chinese medicine. Dry needling is more of a Western medicine approach.