In the context of heat pumps, a "backup heating strip" is a resistive heating element - electricity goes in, heat comes out just like an old electric heater. They're generally used during defrost cycles for the external air handler in cold weather, or to provide a temporary thermal boost when the heat pump is having performance trouble e.g. when the exterior temperature is near the bottom of the heat pump's operating range.
More detail at https://carolinacomfortsc.com/hvac/what-are-heat-strips/ (no affiliation, they were the first authoritative return from my web search).