Rotoscoping is the process of manually altering film or video footage one frame at a time. The frames can be painted on arbitrarily to create custom animated effects like lightning or lightsabres, or traced to create realistic traditional style animation or to produce hold-out mattes for compositing elements in a scene and, more recently, to produce depth maps for stereo conversion.
As a VFX artist, you are primarily creating motion graphics or visual effects. A thorough knowledge of rotoscoping and roto tools is vital to solving a vast amount of problem solving in VFX: rig removal, stereo conversion, greenscreen compositing, hold out mattes, split screens, and even object or feature-based color grading. It is perhaps one of the widely used tools in visual effects.It's a lengthy read, but well worth it. Bookmark it for future reference, too!