A category of data.
Dimensions are like buckets for your data. The values contained in those buckets are names, descriptions or other characteristics of that data.
For example, if you are reporting on website traffic, your data likely contains dimensions like Browser, Landing page and Campaign. The values for these dimensions will contain things like
Summer Sale. Dimensions for geographic data would include Continent, Country, Latitude, and Longitude. The values for these dimensions will contain things like