Using a tank of water as an example, as the pump draws water from the bottom of the tank, the level will fall below the lower sensor. At this time, the valve will receive a command to open and refill the tank. But if the valve opens too slowly, the tank will run dry. When the tank is being filled, the upper level sensor will sense that it is time to turn off the valve. But if the valve closes too slowly, the tank will overflow. This is an example of phase margin, which is a way to measure response time. If the response time is too slow, the system can not keep up with the changes in the level. In this case, it takes so long to turn the valve on or off that the tank runs dry or overflows. For an op amp, if it takes too long for the output to respond to a change on the input, the outputs will oscillate.