Where did this plot come from? Without more information, we won't be able to say exactly how it was created.
However, it looks like this might be a prediction confidence interval plot created by SimBiology. You can find more information about that functionality on the sbiopredictionci reference page. As for the details of how the confidence intervals are calculated, you will see that the function has two ways of calculating the cofidence intervals, described under the "Type" name-value argument. The faster but less accurate approach is the Gaussian approximation, the slower but more accurate approach is the bootstrap method.
