A kill switch is what cuts off your ignition. Without ignition, the spark plug does not create a spark, thus shutting off the engine.
The most ghetto kill switch is pulling off your Spark plug wire while ripping.
The most advanced kill switch is a safety tether kill switch. When you make a whoopsie, and fall off your bike, since it's tethered to you, the kill switch falls off with you. That shuts off the bike and prevents possible sad things from occurring.
The down the middle, most common option is a toggle on your bike, usually on your handlebar. You can't start it unless you turn it on, and you can simply turn it off by... you guessed it: turning the switch off.
If you want to learn how to wire up these bad boys (aside from the ghetto option) is to look up the wiring diagram for your bike and start from there.