This is the standard nomenclature for the faces of the cube:
L (Left) : the side directly to the left of the front.
M (Middle) : the layer between L and R, turn direction as L (top-down).
R (Right) : the side directly to the right of the front.
U (Up) : the side above or on top of the front side
E (Equator) : the layer between U and D, turn direction as D (left-right).
D (Down) : the side opposite the top, underneath the Cube.
F (Front) : the side currently facing the solver.
S (Standing): the layer between F and B, turn direction as F.
B (Back) : the side opposite the front.
By convention, the cube is preferably oriented with:
The red face to the right.
The yellow face on top.
The blue face in front.
As a result:
The X-axis goes from the orange face to the red face.
The Y-axis goes from the yellow face to the white face.
The Z-axis goes from the green face to the blue face.