The x-axis is one of the two number lines that make up a 2D rectangular coordinate system (or one of three in a 3D coordinate system). The x-axis is almost always the horizontal axis and the y-axis is almost always the vertical axis. This is mainly by convention, but in a 2D coordinate system, it is unlikely that you will encounter axes oriented in a different manner. In a 3D coordinate plane however, since there are 3 axes, the x- and y-axes may be oriented differently.
A coordinate plane can be used to describe the position of points in space. The x-axis is used to describe the horizontal position of a point and the y-axis describes its vertical position. The position of a point is written as an ordered pair: (x, y). The first number in the pair corresponds to the horizontal position of a point and is called the x-coordinate. This number indicates the horizontal distance and direction (by convention: negative = left, positive = right) relative to the y-axis.
Points A, B, and C are graphed on the coordinate plane above. The x-coordinate of point A is -3 which indicates that it is 3 units left of the y-axis; the x-coordinate of B is 0, so it is located on the y-axis; the x-coordinate of C is 4, so it is 4 units right of the y-axis.