Line wires are high voltage wires and load wires are low voltage wires.
In a power distribution system, they’re often drawn as thick red lines and thin blue lines. Line transmission is typically at a set nominal current, usually 13,000 volts for USA systems, which can be thought of as constant amount of electricity that the wire will carry on its own without any other contributions such as transformers or reactors to increase the generated voltage. Load transmission is at a much lower nominal current than line transmission and is essentially just enough to operate an electric device with no unneeded energy left over when done operating the equipment. Loads need transformers or reactors because it’s hard for them to get their electricity needs met by just one wire due to the losses in wiring.
