To save you arriving at HotRIBs then casting round wondering what on earth to click on next, here are some direct links:
Trimming devices part 1
Trimming devices part 2
If your RIB is porpoising it generally means that it is not set up correctly, and is not running as efficiently as it could. It may simply be a matter of having the engine trimmed too far out, or it may be something more fundamental in the setup of the boat.
At lowish speeds it's not a big deal (although annoying) at high speeds it can become dangerous as the oscillations grow in size and more control is lost.