The formula is originally v = d/t 

It shows how velocity is an object's displacement over the time it takes for it to move that far. 

If you multiply both sides of the equation by t, you get this:

vt = dt / t

But the t on the right side of that cancels, so you get vt = d 

Or, d = vt. 

However, sometimes you only need to consider the horizontal displacement of an object. So, if something is moving both down and to the right, let's say you only wanted to focus on the rightwards motion of that object. 

You would turn d into dx (as its displacement in the x direction) and v into vx. Note that the x is always in the subscript. 

Using our original formula, that would make dx = vxt