It is a horizontal line of ladder logic.

Ladder logic is a language used to program PLC's.
It's called Ladder Logic because the programs are formed in the shape of a ladder. Each horizontal line in the program looks like the "rung" of a ladder, which is why they call it Ladder Logic.

nmbjbjkjlkkljjgffhk;kh

Programming Logic Controller is a controller, which gets input from sensors,read/limit switch and controls the output according to the Ladder logic program.

Ladder programming is used in Programmable Logic Controllers (PLC's) and is designed to be easily understood by engineers who are used to relay systems. It uses symbols for relay coils, timers and contacts. The advanced capabilities of the PLC e.g maths and logic functions are not used.

Although GM made the initial order with a company called Bedford to develop a electronic replacement for relays which became the first PLC, The "Ladder" Logic which Graphically resembles contacts and relays was first done another company called Reliance Electric.