|z- a|= r can be interpreted as a circle, in the complex plane, with center at a and radius r. In particular, is a circle with center at and radius . The set is the interior of that circle. Similarly, is a circle with center at and radius . There will be at least one point on that circle, in the interior of the first circle if and only if those circles over lap. That is true if and only if the sum of their radii, , is greater than the distance between the centers, .
That is, there will be at least one such complex number as long as .