Tolerance:

The capacitors are rated according to how their values can be expected to match the rated capacitance. The common tolerance is 10 percent; some capacitors are rated at 5 % or even at 1 %.  In all the cases, the tolerance ratings are plus-or-minus.

Thus, a 10-% capacitor can range from 10 % less than its assigned value to 10 % more.

Problem : 

A capacitor can be rated at 0.001 µF, plus-or-minus 10 %. What is actual range of the capacitances it can have?

Multiply 0.001 by 10 % to get the plus-or-minus  variation. This is 0.001* 0.1 = 0.0001 µF. Then subtract and add this from the rated value to obtain the maximum and minimum capacitances which are possible. The result we obtain is 0.0011 µF to 0.0009 µF.

You mayu prefer to work with the picofarads instead of microfarads, if small numbers make you feel uncomfortable. Change 0.001 µF to 1000 pF. Then the variation is plus-or-minus 1000 * 0.1 = 100 pF, and range becomes 1100 pF to 900 pF.