The primary function of a dimension is to provide grouping, filtering and labelling abilities. Google Analytics reports consist of measurements that indicate the performance of website in terms of numbers. These numbers are characterized in two primary categories - Google Analytics dimensions and metrics.
Dimension is a descriptive measurement and is used for data that cannot be assigned a numerical value. For example, a dimension is a city or a country the visitor comes from. Another example of dimensions is page, where value is URL address of the page. Basically, anything that cannot be counted and expressed in numbers in Google Analytics is a dimension.
It is possible to create custom dimensions, such as categories of products or authors of the posts. Also, combinations of dimensions and metrics are possible, such number of sessions originating from certain country for example.