What is a Short?
For all electrical circuits, a "short" is a short-cut that the electricity is jumping across.
Usually, a "short" is thought of as one wire touching another wire. The short accidently connects the wires together so electricity flows freely from one wire to the other. When the electricity flows freely from one wire to the other, the electricity doesn't reach the end of the circuit - it has taken the short-cut instead.
A switch is a connector. When it turns on, it closes its contacts together; it connects two wires so the electricity flows freely from one wire to another. There is no difference between a turned-on switch and a wire-to-wire short.
A relay is a switch. Instead of a person pushing a button or lever, an electromagnet pushes the button or lever. When it closes its contacts together, it connects two wires so the electricity flows freely from one wire to another. There is no difference between a relay's closed contacts, a turned-on switch, and a wire-to-wire short.
Fire Alarm System Short
On a conventional fire alarm system, a beam detector is a device that is connected to the fire alarm zone input. When the beam detector goes into alarm, it "sends" the alarm to the fire alarm system by turning on its alarm relay. The relay then "shorts" the fire alarm systems detection zone wires together.
A flow switch shorts two wires together, so does a pull station. a tamper switch, low air switch, and many other types of inputs for fire alarm inputs.
On the other hand, a short on a fire alarm system's output, like a Notification Appliance Circuit (NAC) is bad. A short there can prevent the fire alarm system from doing its job of warning people that there's a fire.
Because the fire alarm system can't tell the difference between a wire-to-wire short, a short caused by a turned-on switch, or the beam detector's alarm relay, the fire alarm system can only tell you that there's a short.