In an effort to reach a state of stability, substances that are lacking electrons seek out electrons wherever they can: these substances are referred to as oxidizing agents.  On the contrary, substances which have a surplus of electrons are capable of donating their extra electrons: these substances are referred to as reducing agents, or anti-oxidizing agents. Oxidation-reduction potential, or ORP, is a measurement that indicates the degree to which a substance is capable of oxidizing or reducing another substance.
ORP is measured in millivolts (mV) using an ORP meter.  A positive ORP reading indicates that a substance is an oxidizing agent.  The higher the reading, the more oxidizing it is.  As such, a substance with an ORP reading of +400 mV is 4 times more oxidizing than a substance with an ORP reading of +100 mV.  A negative ORP reading indicates that a substance is a reducing agent.  The lower the reading, the more anti-oxidizing it is.  As such, a substance with an ORP reading of -400 mV is 4 times more anti-oxidizing than a substance with an ORP reading of -100 mV.