A resistor is a component that introduces resistance that opposes the flow of current.
A resistor has a set relationship with voltage and current, shown below
V = I * R
When a resistor is in series, the values add. This means the value will always increase.
When a resistor is in parallel, the resistance is the reciprocal of the sum of the reciprocal. This means the values will always decrease. For example, for R1 R2 and R3,
Total = 1/(1/R1 + 1/R2 + 1/R3)