My Canon speedlites (like the Nikon flashes) have an “Autofocus Assist Mode”, where a red pattern of lines emerges from the red square at the front. I am talking about big flashes, like my 580EX 2:

580ex2 Flash

The flash emits the red beam when needed to help the camera focus. This happens:

1. When it is dark
2. When the subject has too little contrast (try focusing on a blank wall).

It is cleverly linked to the focus spot: when you are using a focus spot that can detect horizontal lines, it will emit horizontal lines; when using a focus spot that can detect vertical lines, it will emit vertical lines.

But when does it not work?

1. When it is not dark and the subject is not low contrast. If the camera can focus without the assist light, it will. Why waste battery power?
2. When you are not using “One Shot” focus (Nikon calls it AF-S). The red light helps the camera lock focus: when you are in a mode that does not lock (namely manual focus or AI Servo/AF-C focus), it will not try.
3. When you are using a focus point that cannot be reached by the red pattern (think wide angle lens).

Hope that helps. Flash can seem very complicated, but if you know all these little things, it gets a lot easier to use.