Resistor tolerance is how accurate the resistor value is. You
may have a 10% tolerance, 820 ohm resistor, which means the actual
value of resistance is 820 ohms +/- 82 ohms.
If you design a circuit that has very stringent requirements for
resistor values, you may need to purchase a resistor that has a
smaller tolerance (like 5, 2, 1, .5%). The smaller the tolerance,
the more expensive it will be (generally).