Having created several mobile apps, there are 2 main types of "help":
Often the first thing the user sees, even before signup (if needed) or initial usage. It serves to explain what the app is about, its main appeal and capabilities.
It usually takes the form of several swipable pages, or a short video.
On-screen tips and pointers
Much more specific help, offered exactly in the place the user might need it. Usually appears with new UI elements that might not be completely self-explanatory, or when the developer wants to draw the user's attention to a less obvious (and hence important) action/flow.
Usually both appear only once and can be accessed again from the "settings" or "about" pages.
In general we use our mobile phones for brief periods of time, launching apps that are most of the time very straight forward and simple. Any app that is not self explanatory and intuitive is immediately discarded. No one would sit around to actually read anything to help them figure out what the app is about. I agree with Ben - I rarely read any document provided with any piece of software and if I have to, it's usually a bad sign. Without a doubt though this is accentuated with mobile apps.