Tolerance in engineering is an allowance made for imperfections in a manufactured object. For example, an electrical specification might call for a resistor with a nominal value of 100 ohms, but should also state a tolerance such as "+/- 1%".
This means that any resistor with a value in the range 99 ohms to 101 ohms is acceptable. It would not be reasonable to specify a resistor with a value of exactly 100 ohms, because such a resistor cannot be made. It is good practice to specify the largest possible tolerance, to make manufacturing easier and to keep costs down.
Source: Wikipedia
