Notation is essential to understand in ordr to solve the cube. Notation is what allows to clearly communicate a move on the cube without having to show it on a cube.
When you read a side's corresponding letter, this instructs you to turn that side a quarter turn clockwise. If the letter is followed by a " ' " (read as "inverted"), it represents a quarter turn counterclockwise. Also, when a "2" follows the letter, it is a half turn of the face specified.
Here is a cube, with the letters for the faces on the corresponding centers.