The Canon 430EX ii Speedlite features a Guide Number of 43m measured at ISO 100.
The Guide Number is a measurement of flash output power and indicates at what distance the flash would provide good illumination if an aperture of F/1 was used.
This means the Speedlite 430 EX ii has sufficient power to be effectively used on subjects 10-15m away, if a sensitivity of ISO 100 is used.
This distance can be doubled or quadrupled by using a higher sensitivity.
Plenty of power also means that the flash light can easily be bounced off the ceiling or used with a softbox, producing a soft light yet still provide adequate illumination.
This makes the Canon 430EX ii the perfect tool for indoor flash photography such as weddings, events and studio or location portraiture. If you need a flash for outdoor flash photography like wild life or sports, you should consider looking at a flash with more output power, like the Canon Speedlite 600 EX-RT.