Climate is a long-term view of weather. A climate description of a region is an average of its weather factors: temperature, rainfall, etc. These factors are averaged over decades—usually 30 years—to describe the climate of a region.
It’s useful for gardeners to know their climate so they can select plants that will thrive there. All plants originated in a specific climate, and have evolved to cope with that climate. They “expect” certain conditions. Desert plants, for example, have evolved the ability to go for long periods without water, then to make the most of infrequent rainfalls. On the other hand, since many fungus diseases only exist in moist soil, they have little ability to defend themselves against the fungi that cause root rot, so rot easily in moist soils.
You don’t have to select plants native to your region—only plants native to regions with similar climates. For example, much of California has a Mediterranean climate, with hot, dry summers and mild, wet winters. Mediterranean climates are also found in the lands bordering the eastern Mediterranean, the west coast of southern Africa, the west coast of Australia, and the west coast of South America. Plants from any of these regions can be grown in California with a minimum of fuss.
The United States Department of Agriculture has published a Plant Hardiness Zone Map that uses average winter low temperatures to identify where plants can grow.