Often a horse's natural reaction to something that it doesn't understand is to spook or shy. A spook is usually a startled jump sideways, or a quick change of direction with the intention to flee. The horse may or may not want to keep their eyes on the object that frightens them. In the wild this quick reaction is a response that would allow a horse to flee a predator very quickly. Riding a spook can sometimes be fun—but often it is annoying and even dangerous if you come unseated. A horse that spooks in hand can knock you or others down, which can be very dangerous. For a beginner, a spooky horse isn't the best choice as being surprised by the startled reaction and quick movement of the horse may be unsettling and confidence eroding. A violent spook may land you on the ground, and nobody enjoys falling off.
It's almost impossible to avoid every spook, but some horses are more likely to be reactive then others. As a beginner, you'll probably be looking for a bombproof horse, one that has seen and done it all. However, that doesn't mean your horse will never spook or shy. The quietest of horse can still react to something that surprises it. In the wild, this quick reaction response was what kept horses from the jaws of predators. Even though horses have been domesticated for a few thousand years, they still retain this very natural tendency. Sometimes there are situations where you encounter something there's simply no way to prepare for. I remember when two hot-air balloons floated above our paddock. The horses, galloped around in fear, snorting with their eyes bulging. It only happened once in ten years, and I'm really glad I wasn't riding at the time!
While spooking is a natural reaction to being startled, some horses that are high energy will spook to burn off steam. A horse that is uncomfortable with a badly fitting saddle, too tight girth or other physical pain such as chiropractic issues may be 'spooky' in response. Spooking may also be an indication of vision problems. Some horses are more insecure than others, and if they don't respect the handler or rider as a leader, they won't trust them to keep them away of unsafe situations.
Ideally, your horse will have a blasé attitude whenever you're riding or handling it, but the extreme opposite of this is the horse that seems to spook at every trembling leaf, change in light and shadow, patch of daisies, or unexpected rock or tuft of weeds. This is very unnerving for the beginner rider and can make the spooking worse. Horses are emotional sponges, and if they sense the rider or handler is nervous, they will pick up the negative energy. Often horses that are relaxed when turned out in the ring or pasture will suddenly find things to be frightened of when they are ridden in those same places. This is because they are sensing the rider's worry and becoming worried themselves. It turns into a vicious cycle as horse and rider each makes each other more insecure.
If your previously quiet horse has become progressively spookier, you first need to look at any possible physical problems—chiropractic, painful tooth issues, saddle fit or vision problems. If you're feeling nervous, a good coach or instructor can help you work past confidence issues. If your horse is afraid of specific things—like mail boxes, pots of flowers, or puddles of water, your coach can help you desensitize your horse. I wouldn't recommend that a beginner try desensitizing a horse on their own, because done incorrectly it is possible to make the problem worse. I've seen people try to desensitize their horse to flapping plastic bags by tying a bunch of bags around their stall, only to have the horse become afraid of being in the stall. One problem became two.
The better schooled your horse is, the more you will be able to control his reactions when he spooks. By applying leg aids, you may prevent a spook from turning into an 180 degree spin. However, first your horse has to be taught to be respond to leg aids. This again is where good coaching can help you learn to have effective seat and leg aids. Many horses will spook in one direction. If my mare spooks, she'll most likely move to the right. She's caught me day dreaming the odd time, and made me appreciate having a good following seat. So, it's important not to zone out completely when you ride. When I sense she's about to spook, along with applying a little reassuring seat and leg to move forward, I'll keep my right leg on her, so she doesn't push through it. The better rider you become, the less likely your horse will spook.
On the ground, your horse should always know to keep its distance. It should understand that it is never acceptable to initiate contact (as I mentioned in Horses that Bite). Again, desensitizing exercises with a competent coach can help prevent spooks or shys on the ground or in the saddle.