It's about an investigation carried out on the transformer effect . The primary coil of 20 turns and the secondary coil of 50 turns is wound around a laminated iron core.
For an applied AC voltage, the primary coil has an impedance from self-inductance which limits the current (amperes) flowing through the copper (unless you draw current from the secondary coil).
For an applied DC voltage, the impedance from self-inductance is zero, which causes a large current to flow. This current heats up the wire with a power $P=I^2R$, where $I$ is the current and $R$ the resistance of the wire. The heating is much smaller in the AC case because $I$ is much smaller in that case.