Inside every electric motor, copper windings carry current. They sit inside steel slots. Steel conducts electricity. Copper conducts electricity. If they touch, current leaks. The motor shorts. Performance drops. Eventually, the motor fails.
The only thing standing between the copper and the steel is a thin sheet of material called electrical insulation paper.
It does not look like much. A fraction of a millimeter thick. Cut into precise shapes. Slid into the slot before the windings go in. But without it, the motor does not work.