There are two types of lights, one to be seen with and the other to see with. The first are the types of lights you should have if you are riding on well-lit roads. You need the latter if you are riding on remote country roads after dark and need a light so you can actually see where you are going.
The majority of commuters need lights to be seen with. These lights just need to be strong enough to alert other road users of your presence and ensure you are easily seen.
Light brightness is measured in lumens. Most manufacturer’s commuter lights offer between 50 and 300 lumens. You do not want to opt for the lower end of this range as even on brightly lit streets, the light will not be very effective.