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The Brush Editor dialog allows you to make alterations to brushes. At the top of the dialog, you can see the name of the brush (a new brush will be called "Untitled"). This name can be edited to change the brush name. If you choose a name which is the same as another brush, GIMP will suffix the name with #x. 
Below the title, you can see a preview of the final brush. If you size the brush to a size larger than the preview area, the preview will be scaled and the scale factor shown below it as a ratio, such as 2:1 for 50% zoomed out.
The remainder of the dialog controls how the brush will look. The Radius controls the distance from the center of the brush to the furthest edge in pixels. The maximum radius of a brush is 100 pixels.
The Hardness slider controls how much the brush fades towards its edges. A value closer to 1 will give a sharp edge and closer to 0 will give a fuzzy, blurred effect.
The Aspect Ratio defines the vertical diameter in a ratio to the horizontal diameter. A value of 1.0 will give a round brush since this means that the horizontal diameter is the same as the vertical. The maximum value is 20.0 which gives a horizontal diameter twenty times larger than the vertical, resulting in a very squashed, elliptical brush.
The final setting is Angle which is used when the aspect ratio is more than 1.0. The angle can be between 0 and 180 degrees, measured counterclockwise.