You have two sensors, P and Q, used to measure some property of two entangled particles. When P and Q have the same setting, they always disagree, because the e

You have two sensors, P and Q, used to measure some property of two entangled particles. When P and Q have the same setting, they always disagree, because the entangled particle that Q measures has a state opposite to the particle measured by P. Similarly, when Q has a setting opposite to that of P, P and Q always agree. For any other setting, P and Q will sometimes agree, and sometimes disagree. The extent to which the agree is what quantum physicists refer to as the correlation. If they always agree, the correlation is 1; If they always disagree, the correlation is -1; when they sometimes agree you'll get a value between -1 and 1. It's really just the percentage agreement between the two detectors.