The keyword represents an expression that you want to track on a search engine.
A keyword can be tracked on several search engines / devices / locations, so there are as many keywords consumed as there are combinations chosen.
Example:
You track a keyword on google.fr and google.co.uk: 2 keywords are counted
You track a keyword on google.fr (Paris) and google.fr (Marseille): 2 keywords are counted
If you track a keyword on desktop and mobile: 2 keywords are counted