When estimating the mean material needs for a construction project you are looking to determine the average amount that will be needed. This is extremely useful with dealing with materials that are easily arranged or that flow. Cement, mortar, gravel, rocks, and sand are on the top of the most lists.
The amount of material in a given area may vary but the average or mean amount of material for a larger area should be within a normal range.
Take pouring a sidewalk as an example, because the base lay of material will not be perfectly flat and solid the amount of cement on top to create a smooth level walkway will vary. If the base was laid well the variance should not be much, but is the foundation is off the cement thickness will vary greatly.
However, even though the thickness will vary from point to point in the end the highs should equal the lows proving you with an average thickness of material.