In a Cartesian coordinate system, the y-axis sits at a 90-degree angle from the x-axis. The two axes meet at a point where the numerical value of each is equal to zero. On a map of the world, this is analogous to the point where the equator is crossed by the prime meridian.
The x-axis is essentially an infinite number line running horizontally in both positive and negative directions outward from zero. With the use of such a number line, any number can be found and plotted. The y-axis is a similar number line, only running vertically and perpendicular to the first. By assigning a value for both x and y, any point on a two dimensional plane can be plotted. These values for x and y are called coordinates and given in pairs with the x value first. A point located three units to the right of the origin on the x-axis and two units above the origin on the y-axis is at the coordinates (3,2).
The Cartesian coordinate system is highly useful in mathematics and has many practical applications from engineering to map making, but what it represents is arbitrary in real terms. There is no reason, for example, to assign the directional axis that goes north-south to y, or east-west to x, except that it is conventional to do so.