Davey,
Sometimes it helps to hear from someone who was recently clueless rather than the usual field of true experts. We can relate better to what it means to be a true novice. The tilt function raises and lowers the engine when it is not running for launching, trailering, service, etc. But the tilt piston is not strong enough to keep the bottom of the engine from swinging forward when engine is running and the propeller is pushing it forward. The trim piston is larger so it can push and hold the bottom of the engine away from the transom even under pressure from the running engine to get the optimum thrust angle for propelling your boat. If the trim system fails, the bottom of the motor will simply swing forward as far as the hardware will allow and stop. You won't get the optimum thrust angle, but its otherwise no big deal. The suggestions above relate to adjusting the hardware to get a better, but not easily adjustable trim angle. On my pontoon boat, the trim angle made no discernable difference.