Two or more resistors in series can be used to set a required voltage at a point in a circuit. For example, this could correspond to a threshold voltage at which an alarm should sound or perhaps it is the voltage output required from a power supply circuit.
The voltage is divided in the ratio of the resistor values (assuming the current is the same in both resistors).
where Vs is the supply voltage.
By Ohm's Law
or ...
A potentiometer (pot) is a variable resistor with an adjustable slider. You can think of it as two resistors (one on each side of the slider). The diagram below shows the commonest arrangement. There is another straight/linear version often called a fader. Both types are used as volume and tone controls. They are notorious for being unreliable. Dirt gets into the moving parts and corrosion causes problems.