Porpoising is when the boat keeps undulating up and down, you know, like how a porpoise or a dolphin does when swimming up and down out of the water like they do when playing running along in front of a boat sometimes.
The ride plate is another ride tuning tool that you just have to play with to find the sweet spot, and yes it will help keep the nose down if you lower it but remember anything it makes your boat do at slower speeds it is also going to do too a higher degree at faster speeds. Do you have a diverter? If you don't I would find the plate setting that allowed you to maintain a happy cruising speed without porpoising but still allow the boat to run happy on the big end without plowing the nose.