The small invisible lines that connect power using units like lights and doors with signal units that controls the system.
Press F2 to get a view of where electrical using units is wired up with signal units. Edit signal lines by connecting units with right click and remove signal units (drill) to remove the connection.
Signal Lines are invisible (when F2 isn't avtive) and can go through blocks. There are a limited length on Signal Lines but can be extended with a Repeater.
Signal Lines is different from Power lines