Both terms relate to film budgets.
“Above the Line” costs are generally all of the costs involved in a production before principle photography begins. These are not necessarily all pre-production costs, as some of these may relate to the shoot itself. Rather, above the line costs include things such as salaries for the producer, director, and sometimes big name stars, producer’s expenses in getting the project up and running, and fees for screenwriters and any underlying rights for the script.
“Below the Line” costs are basically everything else that doesn’t come under “Above the Line.” This includes the cost of the crew, most cast, shoot, catering, coffee, post-production etc.