2 possible notations in the algorithms for a mini, rubiks, revolution, and professor cube. In other words it's termonology for the 2x2, 3x3, 4x4,....AxA rubik's cube.
There are other notations such as R and L (right and left). Adding a lover case I (i) after each notation means it is the inverted notation (opposite).
F R U Ri Ui Fi is an algorithm for the rubik's cube. F U are 2 notations in the algorithm.