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)