Check the picture below.
the distances d1 and d2 are for this ellipse 7 and 3 respectively, keeping in mind that their sum is always the same, namely d1 + d2 is always the same 10 no matter where on the ellipse you move the point, notice in the picture, if we move d1 to the extent that lies flat right on top of d2, we have the length of d1 in purple, plus the length of d2, which we later colored in darkyellow, if we move d2 to the right-hand-side or the tail of d1, we'd end up with the ellipse's major axis, which is oddly enough, d1 + d2.