As kvbarkley noted, this series of speedlights isn't like studio strobes with an integrated light sensor to fire the strobe based upon the flash output of another strobe but instead are controlled by an infrared signal either from a cameral mounted speedlight set to master or an ST-E2 transmitter that mounts in the hot shoe and triggers one or more remote mounted speedlights.
Because it uses an infrared signal, similar to many remote controls, the lights need to be fairly close to one another and there needs to be a good either direct or bounce/scatter path between the controller and remote strobes. Range can be very limited outdoors because any light soure with strong infrared content will desense the receiving unit(s). The positive is that these flashes won't be accidentally triggered by someone else using their flash UNLESS it is another Canon unit close by and set to the same channel.
I used a ST-E2 transmitter with 580EX and 420EX flashes for several years and found the Canon control system is pretty reliable as long as you ensure that there is a reasonable light path between controlling and controlled units. As the RF spectrum becomes more polluted at many events with various WiFi/bluetooth/etc. devices these older infrared units start to show benefits over the more recent RF controlled units that have to contend with a lot of interference from an ever increasing number of devices.