Glossary
Dart
A dart is a wedge folded out of a flat pattern so the fabric takes a three-dimensional shape over the body.
Cloth is flat and bodies are not. A dart is the most direct way of resolving that: remove a wedge, stitch the two legs together, and the flat piece cups.
A dart has an apex and two legs. The apex points at the fullness it is shaping and stops short of it — a bust dart that runs all the way to the point produces a cone, which is why the apex is normally set back one to three centimetres.
Darts can be moved anywhere around a piece without changing the fit, as long as they still point at the same apex. That is dart manipulation, and it is most of what makes one bodice pattern into a hundred different styles.