Machine tolerance is the set of values used to identify the allowed variance from the stated dimension. The machine tolerance is most often seen on the part drawing used in machine shops when crafting items made from metal or plastic.
Tolerances are stated as a plus/minus value. For example, a part made from 1-inch diameter round stock could have a tolerance listed at +.010"/-.010". As the overall size of the part increases so do the standard tolerance variances.
Due to a variety of differences in the machining process including source material, machine operations and wear on machining tools, it is nearly impossible to produce exactly the same part. Tolerances are used to allow for these variables.