Final answer:
The production function describes how much output a firm can produce with different inputs. Each product has its own production function, and firms in the same industry may have slight differences in their production functions. Fixed inputs are those that can't easily be changed, and they define the firm's maximum output capacity.
Step-by-step explanation:
The production function gives the answer to the question, how much output can the firm produce given different amounts of inputs? Production functions are specific to the product. Different products have different production functions. The amount of labor a farmer uses to produce a bushel of wheat is likely different than that required to produce an automobile. Firms in the same industry may have somewhat different production functions, since each firm may produce a little differently. One pizza restaurant may make its own dough and sauce, while another may buy those pre-made. A sit-down pizza restaurant probably uses more labor (to handle table service) than a purely take-out restaurant.
We can describe inputs as either fixed or variable. Fixed inputs are those that can't easily be increased or decreased in a short period of time. In the pizza example, the building is a fixed input. The restaurant owner signs a lease and is stuck in the building until the lease expires. Fixed inputs define the firm's maximum output capacity. This is analogous to the potential real GDP shown by society's production possibilities curve, i.e., the maximum quantities outputs a society can produce at a given time with its available resources.