The probability plot shows a graph with observed cumulative percentage on X axis and expected cumulative percentage on Y axis. The Weibull probability plot is used to test whether or not a dataset follows Weibull distribution. And its X scale type and Y scale type are Log10 and Double Log Reciprocal respectively. If all the scatter points are close to the reference line, we can say that the dataset follows the Weibull distribution.
You can refer to this graph in Learning Center. (Select Help: Learning Center menu or press F11 key , and then open Graph Sample: Statistical Graphs and search "weibull")