In orthodontics we move teeth with basic mechanics. When you push on something (a tooth), you have to pull on something else. You "brace" yourself against a stronger object. When you are moving teeth, the thing you brace against is called the anchor-you know, like an anchor on a boat. Most of the time, anchorage is achieved by placing one force against a small tooth that you are trying to move, against another large tooth (or several teeth tied together, the "anchor"). The small tooth moves a whole lot more than the anchor. There are several instances in orthodontics where this anchorage is an elusive thing to acheive. TAD means Temporary Anchorage Device. These are basically very small screws inserted into bone. The TAD does not move, the tooth then can be moved against this fixed point. TADs are removed when they are no longer needed.
Wait, a screw? in bone? Ouch! doesn't that hurt? Well, --no. We even tried this on each other before using it on patients. Topical anesthetic does great for placement. Here is one of our docs with a TAD in place: