Most likely it means that you have forgotten to turn in an assignment. The zero serves to let you know that I have graded the assignment and have not received yours. Once you turn this assignment in, the zero will change and you will receive partial credit. There's also always the chance that I have made a mistake and forgotten to enter your grade. In this case, please let me know and I will gladly change your grade.
Do you accept late/missing work?
Yes! As long as the late work is turned in within the grading period, I always accept late work for partial credit. I want you to learn, and if you do the work then you should get credit for it.