"Time isn't the main thing, it's the only thing."
On a short film, a production manager (aka line producer or PM) is responsible for most aspects of the production, including scheduling, shot lists, storyboards, talent & crew call sheets, budgeting, and script breakdowns, but most importantly the management of time. Although this list is not exhaustive.
The PM's job is to organise the shoot so that it goes without a hitch, on time, and on budget. Working with the producer, director and other heads of department the PM will create reports and communicate with the talent and crew to arrange the shoot: times on set; pick up times if appropriate; and keeping the shoot on overall schedule. In the short film environment where there is no monetary budget as such, it is time that is used as a commodity.