Notation is used to notate algorithms, which are sequences that help you solve the rubiks cubes. In notation, every face turn has a notation.
The 3x3 Rubik's cubes notation includes six basic face turns, which each have various suffixes which modify the turns. F means to turn the front face 90 degrees clockwise, while F' (F Prime)means to turn it 90 degrees counter-clockwise. Likewise, B means back face and R means right face. L means left face, U means up face, and D means down face. This chart should help clear things up: