Something substandard is below standard — it’s shoddy or inferior. If your final paper for honors English class is scribbled on a torn sheet of notebook paper, that’s substandard work that you’ll probably have to do over.
A standard is a quality by which something is judged, and sub can mean “under” like a submarine that goes under water. So things that are substandard fall below most people's standards — they're deficient in some way. A computer that stops working after two weeks is substandard. A car with faulty brakes is substandard. Substandard things are the opposite of well-made things. Turning in substandard work is a good way to get a bad grade.