I need HELP ASAP

Mrs. Hool is planning on grading tests over the weekend. The number of tests that she has left to grade after each hour can be modeled by the equation = 78 − 15ℎ, where is the number of tests left to grade and ℎ is the number of hours she has worked on grading. What is the meaning of the values 78 and −15 in this equation?