RMS measures the overall roughness of a surface by taking a number of surface irregularity readings and then calculating the square root of the mean of squares from those readings. Higher RMS numbers indicate rougher surfaces, while lower numbers indicate smoother surfaces.
Some objects require a rougher surface, including objects that people hold such as cell phones. If the surface is too smooth, it could slip from a person's hand.
Alternatively, some surfaces, such as seals on medicine bottles, need to be smooth, otherwise the flaws may damage the seal. RMS measurements help determine the necessary roughness a product should have.
RMS measurements only consider the depth of surface irregularities, not shape or direction. Surfaces with these different irregularities but identical depths would result in the same RMS measurement.
Also, multiple RMS measurements do not have a fixed relationship to each other. For example, a surface measurement of 50 microinches is not necessarily twice as rough as a measurement of 25 microinches.