Sidelight can be explained fairly simply. It's the direction of light that strikes your subject from the side, or rather, from a 90 degree angle approximately. Read that again, it strikes your subject, not YOU! This means if we think on a three dimensional level, that "sidelight" can actually come from above instead of just the left or the right depending on the position of your subject. Noonday sun can even act as sidelight. The direction of the light can greatly affect how your subject appears in the image. It can bring out textures otherwise missed on a front-lit subject. It can create a feeling of depth by adding contrast to your scene, resulting in a more 3 dimensional feel. Test it for yourself. The next sunny day, find a tree in full sun. Look at the bark from the front lit side. Very little contrast and the bark appears relatively flat (depending on the tree of course). Now walk 90 degrees and look at the bark as the light strikes it from the side. It appears very different doesn't it. Deeper shadows and more contrast bring out the texture and lines of the tree that are not as easily visible on the front-lit side.

Here are a few examples of side lighting. Walking down the beach about 30 minutes before sunset, the sand just comes alive with patterns and textures that were not visible a couple hours prior. 

The morning fog is bathed in soft sidelight and brings about the subtle textures of not only the fog, but the ridges of the Appalachians here along the Blue Ridge Parkway in western North Carolina.

Here in the Valley of Fire State Park in Nevada (just a 45 minute drive from Las Vegas) the setting sun brings out the lines in the foreground leading to the "Fire Wave", one of the most popular sunset spots in the park...for good reason. 

Being a nature photographer is more about studying the light than studying your camera manual. Tech specs can be memorized, but understanding how to best utilize the light available to you requires time, practice, and keeping your eyes open to all the possibilities.