the level of detail of an image

http://en.wikipedia.org/wiki/Resolution

RGB

The RGB colour model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colours. The very idea for the model itself and the abbreviation "RGB" come from the three primary colours in additive light models.

 http://en.wikipedia.org/wiki/Main_Page

To help you get your bearings, you can choose to display Rulers. [View --> Rulers] Rulers appear along the top and the left side of the Stage.The ruler is measured in pixels.