An axis is a a fixed reference line for the measurement of coordinates.
Fortnite takes place in three dimensions, so to locate something in the space, you need three points of reference, or three axes. These points are called the Left, or L-axis (formerly the Y-axis), the Up, or U-axis (formerly the Z-axis), and the Forward, or F-axis (formerly the X-axis). Collectively, these coordinates are called LUF.
Imagine you are looking over a flat plane. Place a dot in the center of the plane. This is your 0 point for all dimensions. Draw a line from north to south (away from you and toward you) and make sure it goes through the 0 point. This is F. Now draw a line that crosses at the 0 point, but goes east to west, (left to right). This is L. Draw a third line that also goes through the 0 point, crossing where the other two lines cross, but going straight up and down. This is U. Using those three lines, you can position any object in a three-dimensional space.
X-axis and Y-axis are still used, but only for coordinates limited to two dimensions — such as in reference to UI content. The X-axis refers to a location left of the origin (negative values) or right of the origin (positive values). The Y-axis refers to a location down from the origin (negative values) or up from the origin (positive values).