A residential air conditioning contactor handles two entirely separate electrical power circuits simultaneously. The low-voltage side of the contactor—its small control coil terminals—gets its electrical power from your indoor HVAC system's 24-volt low-voltage transformer via the control wires running from your indoor furnace control board and thermostat. When your thermostat calls for cooling, it completes this 24V circuit, energizing the coil. The high-voltage side of the contactor—its heavy-duty load and line terminals—gets its power directly from your home's main electrical service panel through the outdoor electrical disconnect box, carrying a powerful 240-volt line voltage supply dedicated to running your outdoor compressor and condenser fan motor. You can troubleshoot safely by turning off power. You can verify proper power supply by testing both low-voltage and high-voltage terminals with a digital multimeter.
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