When you have intradermal skin testing done, a small amount of each thing you may be allergic to (allergen) is injected under the skin. If you are allergic to an allergen, you will get a bump and redness where the allergen was injected. After a short time (15-20 minutes), each skin test reaction is measured for swelling (wheal) and redness (flare). A large enough skin reaction is a positive skin test.
Prick skin testing should precede intradermal skin testing as intradermal tests are more sensitive than prick method (i.e., fewer false negatives).