Your incoming service has 2 hot wires and a neutral. At your main service panel you have bus bars that alternate. When you install a double pole breaker you are using a bus bar from each hot wire which gives you 240 volts. You put a meter from one hot wire to the other hot wire and it reads 240 volts. When you use a single pole breaker you use one bus bar which gives you 120 volts. You measure from one hot wire to the neutral/ground which is bonded at a main service panel and it reads 120 volts. Most appliances or equipment over 20 amps is usually 240 volts because the current draw is less.