F U
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.