Copyright protects the expression of ideas, but not the ideas themselves. An idea is conceptual; a product of the mind. Expression is the tangible result of an idea. The same idea usually produces multiple different forms of expression.
There are relatively rare scenarios when expression is not copyrightable. This happens when there is only a limited number of ways to express an idea. In such cases it would be unfair to hold someone liable for copyright infringement when he or she had no choice but to infringe when expressing an idea. Generally, however, expression of ideas is copyrightable.