A zero represents a logic 0 and a 1 represents a logic 1 of the digital line. It's pretty obvious when you have a line changing states which is a 0 or 1 but if a line is at a constant state, it's a little difficult to know whether you're seeing a 1 or 0. You can turn them off by right clicking on the plot in the plot legend
and selecting Label Format>None. You can also do it programatically with the Plot.LabelFormat property.