Different metals have different coefficients of thermal expansion so if they are heated they, expand by different amounts. This means that if two different metals are fixed together side by side, and heated, one side will expand more than the other. The strip will curve as shown below.
If the strip is cooled, the strip will curve the other way as shown above.
The bimetallic strip has many uses. The most important is in a thermostat, used to regulate temperature.
If the room becomes too warm the strip bends, breaking the circuit, so the heater switches off. Then the room cools so the strip bends back, remaking contact. The circuit is complete, current flows and the heater switches back on.