By style we mean the layout of the code—things like how many columns to indent a block of code, whether an opening brace gets indented to the level of the code it encloses, or of the construct that introduces it, or “hangs” at the end of a line.
Most people only need to edit code formatted in just a few well-defined and consistent styles. For example, their organization might impose a “blessed” style that all its programmers must conform to. Similarly, people who work on GNU software will have to use the GNU coding style. Some shops are more lenient, allowing a variety of coding styles, and as programmers come and go, there could be a number of styles in use. For this reason, CC Mode makes it convenient for you to set up logical groupings of customizations called styles, associate a single name for any particular style, and pretty easily start editing new or existing code using these styles.
As an alternative to writing a style definition yourself, you can have CC Mode guess (at least part of) your style by looking at an already formatted piece of your code, Guessing the Style.