An auxiliary contact block is a piece of equipment that contains auxiliary contacts for a relay. A relay is a type of switch that is operated electronically, commonly with an electromagnet.
Every relay contains contacts, which are pieces of electrically conductive material, usually metal. When a pair of contacts touch, they pass an electrical current, making the switch 'closed.' When the contacts separate, the switch is 'open,' and the current ceases to flow.
In electromechanical relays, auxiliary contacts are often used in a relay logic circuit, or for some other part of the motor control scheme, typically switching 120 Volt AC power instead of the motor voltage. One contactor may have several auxiliary contacts.
Auxiliary contact blocks are used with specific relays, such as thermal overload relays and motor starter protectors. They are also differentiated by their contact configurations, such as DPNO (double pole normally open) and DPDT (double pole double throw); and in the way they are mounted, such as through a chassis mount or DIN rail mount.