Cube Notation is letters and letter character combinations to describe specific turns on a 3x3 Rubik's cube.
The purpose of notation is to easily describe an algorithm or a scramble.
Uppercase letters describe one 90 degree turn of one layer the letter represents.
If the letter is followed by an apostrophe then the turn is anti-clockwise.
Lowercase letters means you turn 2 layers simultaneously. So you are turning the face and corresponding inner layer.
Outside Face Turns
Inner Layer Turns