The Wake-on-LAN feature involves sending magic packets to properly-configured devices that support Wake-on-LAN. When such a device receives such a packet, it will wake the device out of its sleep or standby mode.
Note that, on older hardware, properly configuring a network interface card (NIC) can involve attaching a special Wake-on-LAN cable to the motherboard, and then enabling Wake-on-LAN in that machine’s BIOS.