Coordinate all technical aspects of a live theatrical production.
A Line Producer manages the budget for a film or TV set. You help hire the crew, oversee the budget, and keep all the various departments in line.
For a visual image, think of these sets as assembly lines. Film Producers concern themselves with the bigger picture: Renting the factory, hiring the Managers, and so on. The Film Director, on the other hand, conducts the assembly line and dictates how the pieces get put together. As a Line Producer you are a liaison between the two, working with the Director to make sure all the parts are cost appropriate, and that the assembly stays on schedule.
On a film set your duties as a Line Producer are very similar to those of a Unit Production Manager (UPM). So similar, in fact, that on smaller productions only you or a UPM will be hired, and on large productions you end up overlapping a whole lot.