In film and video, the production phase refers to the physical process of filming. This is what you'd imagine it to be — actors on set, lights, camera, a-c-t-i-o-n! The production phase is also referred to as principal photography. While this phase is typically the most exciting, it's also the most demanding; this is where you have to get it right. And thanks to all of your prep work in the pre-production phase, you're prepared. But... (there's always a but).
Even with all of the planning, and re-planning that you did in the pre-production phase, something can and will go wrong. A light will break, a piece of gear will fail, you'll be hit with a freak hurricane while filming in Iowa - you name it and it can and will happen. But there's good news: And thanks to all of your prep work in the pre-production phase, you're prepared (heard this part before?).
For most big-budget feature films, the beginning of the production phase denotes the "point of no return"; the point at which it is no longer financially viable to cancel the project. At this point it's usually cheaper to continue until the project is finished than to deal with the financial fall-out of canceling the project.
The goal of the production phase is obviously to film all of the required shots, however it's common to film "pick-up" shots in the post-production phase. Pick-up shots are usually required when a mistake is noticed, a script change is made, or even if a performance is deemed to be unsatisfactory.
Ready for More? Checkout: What is Post-Production?