The Hatch angle, as the name suggests, is the angle at which the Hatch is made in an area. Let's take the example of the ANSI31 Hatch pattern. By default, the Hatch is aligned to an angle of 45 degrees with respect to the x axis, and if applied in a simple rectangle, it looks like this with the default setting:

Now, if you want to rotate the Hatch pattern so that it becomes completely vertical, then you can add a Hatch angle of 45 degrees. The Hatch angle starts from the default angle of the Hatch pattern, so in this case, since the ANSI31 pattern is already at an angle of 45 degrees, adding an Angle of 45 in the Properties panel, as shown in the following screenshot, will make the total angle 90 with respect to x axis:

The final Hatch pattern will become vertical, as shown in the screenshot here:

Similarly, you can add a Hatch angle of -45 to the ANSI31 Hatch pattern to make it horizontal. All the other Hatch patterns except the Solid Hatch pattern also behave in a similar way.