What is the purpose of `fliplr` when using the `fill` function for error bands in MATLAB?

The `fliplr` function (flip left-right) is crucial when using `fill` to create a closed polygon for an error band. The `fill` function expects the vertices of a polygon in order. After defining the x and lower bound points, and then the x and upper bound points in forward order, `fliplr(x)` and `fliplr(upper_bound)` are used to trace the upper boundary back to the starting x-value, effectively closing the shape and creating the shaded error band.