On a thick film hybrid, you print resistive paste with a silkscreen. The resultant bulk resistor tolerance is quite wide, and its value is well below the target value.
Then you take your circuit into a specialized fixture, which measures the variable you may want to trim (voltage, current, frequency, etc). A laser starts to cut extremely fine traces into the bulk resistor, increasing its value. This continues until you reach your variable's target value.
The important thing about this procedure, is that the actual resistor value is not that relevant. What matters is the variable you are actually trimming.