Swimming technique drills are are specific movements, done repetitively, to get your technique "in the groove." They can help you get more efficient and they can help you become a faster swimmer. Generally included in all workouts, most coaches feel that you can never do enough technique work. You should include some in your workouts, too.
This list of drills is far from complete. If you are an experienced swimmer, you may either know these drills by different names, perform them somewhat differently, or know many more. Let me know of any that are your favorites.
Important key to freestyle: you spend most of your time on your edge or side, not on your belly! Imitate a sharp knife, on the edge of the blade, not a big soup spoon. Good freestyle, both swimming and drilling, requires you to rotate or roll your body along your "long-axis" or spine. You should also try to take breaths on alternate sides to help promote this good body roll. In these descriptions, if an arm is called the "front arm" it refers to the arm pointing to where you are headed. That side or edge of your body (shoulder to hip) is generally oriented toward the bottom of the pool, like the keel of a boat. The opposite edge (shoulder to hip) is aimed more "up" toward the ceiling (or the sky if you are lucky enough to swim outdoors) like a shark fin.
There are plenty of variations for all drills. You can also combine drills to work on several skills at once, or to add even more emphasis to a single element. Experiment with drills and develop some of your own. Always work to improve your technique.