As found at http://trialscomp.com/observed_trials.htm
What Is Observed Trials?
Trials is a form of motorcycle competition that emphasizes precise balance and control while riding over a designated area of difficult terrain without using your feet to regain or retain balance. It is not a race. Only one rider is "observed" at a time. Throughout the obstacles, speeds can range from fast to very slow. In trials, it is not the speed but the precision that is critical. Every time a rider touches the ground with their foot the observer (judge) scores a point against them.
To better understand this, think of golf. In golf you accumulate points against you and not for you. As you begin each hole, your score, for that hole, is zero. As you play though the hole, accumulating strokes is a bad thing. When you are done with the round, the person with the least number of combined strokes is the winner. In a Trials competition the riders will ride a designated piece of terrain called a "section" starting with a score of zero. Each time they have to put a foot down (dab) they are scored a point by the observer. The rider then moves down the trail to the next section where the process starts all over again. At the end of the competition the rider with the least number of dabs is the winner.
In each section, once the rider has dabbed three times, they may continue to do so without being scored more than three points. A failure to ride the section completely by falling off the bike, going out of bounds or stalling the engine is scored a "5". Five points is the highest score a rider can get against them, in any one section, even if they choose not to ride it for they sake of safety.
The range of skills and techniques employed by the best riders is deceptively varied, subtle, complex and at times brutal. Imagine trying to fling yourself, and your cycle, up and over a five foot stone wall. Or edging down a steep, rocky, moss-covered hillside, then turning across the slope to cross a large wet log before charging and winding your way back up. How about leaping and balancing your way up a mountain stream loaded with rocks, ledges, boulders and waterfalls?
So, Observed Trials is a battle between the rider and the terrain. He who makes the fewest mistakes wins. Since it is a solo battle, a rider can practice at home, even on level ground, perfecting skills such as balance, turning, clutch and throttle control. Trials-type obstacles can even be set up in one's back yard to practice on.
An organizer of a Trials event must discover, design, clear and mark sections. The terrain chosen must be carefully marked with boundaries for each of many set skill levels and must have start and end gates. Small changes in the amount of room prior to an obstacle or in the riding angle across a slope can have a major effect on turning room or traction. So a trials event can be quite mental, not only for the rider, but the organizer as well.
If you add the mental aspect to the physical, note the extraordinary range in skill levels between a Novice and an Expert, and appreciate a wide range of naturally beautiful terrain - then perhaps you will begin to sense the attraction this sport offers its enthusiastic followers and participants.
Thought it was a good explanation, scoring rules excluded.