This is the phase of film production in which the movie is filmed, with actors on set and cameras rolling, as distinct from pre-production and post-production.
The Principal photography phase marks a point of no return for the financiers. While it is common for a film to lose its greenlight status during pre-production (for example, because an important cast member drops out), it is extremely uncommon for finance to be withdrawn once principal photography has commenced, and it is usually regarded as a catastrophe.
Once a film concludes principal photography, it is said to have wrapped, and a wrap party may be organized to celebrate. During post-production, it may become clear that certain shots or sequences are missing or incomplete and are required to complete the film. So additional material may have to be shot. If the material has already been shot once, or is substantial, the process is referred to as a re-shoot, but if the material is new and relatively minor, it is often referred to as a pick-up.