Draft angle: A draft angle is a slight taper on the faces of a moulded or cast part, in the direction the mould opens, so the part can be released without dragging or sticking.
When a part cools in a mould, it shrinks onto the core. Perfectly vertical walls would scrape along the mould as the part is pushed out, scuffing the surface or sticking. Tilting those walls slightly away from the direction of pull, the draft, lets the part come free cleanly.
The amount depends on the depth of the wall, the material and the surface finish. A common rule of thumb for injection moulding is at least 1 to 2 degrees on most walls, more on textured surfaces, because the texture grips the mould. Treat this as a starting point and check with the moulder.
Example: a box 50 mm deep with 1.5 degrees of draft on its side walls is about 1.3 mm narrower at the bottom than at the top on each side. In CAD that difference is invisible from most views, but it decides whether the part ejects cleanly.
Draft applies mainly to moulding, casting and some forming processes. It is not normally needed for 3D printing or CNC machining, so a part designed for printing often has none. Moving such a part to injection moulding later usually means adding draft, which changes dimensions.
A face with negative draft, which leans the wrong way, is a form of undercut. Read injection moulding cost for how design choices like this affect tooling price.
Go deeper: Injection moulding cost: tooling, cycle time, resin and the break-even