What does it mean when you have runny yellow poop?

Hard to say. Many dietary and biliary variations can cause that, without underlying pathology. If this or other bowel symptoms persist, see your doctor.
Diarrhea. It means you have diarrhea and need to change your diet, depending on the severity, you may need clear fluids and gradual increaese in diet in severe case if associated with vomiting, you may need IV fluids and/or hospitalisation it may be caused by viruses, or bacteria like e coli, salmonella, shigella or others most of the time it gets better on its own with change in diet.