This is not at all clear, possibly as a result of being translated to English from some other language.
I think I understand part of it - "point dividing a join of two points in a given ratio". I infer that "join of two points" in your book means the line segment between the two points. If we call the two points A and B, we can call this line segment AB. If P is a point on this line segment, it divides the line segment into two parts that are in some ratio. For example, if P is located at 1/3 of the distance from A to B, then the ratio of AP to PB is 1:2 or 1/2.
With regard to the part about external ratios, I can only guess as to what is meant. For simplicity, let's assume that the points are on the real line, with A at 1 and B at 3. So the line segment AB lies between 1 and 3. If P is at 4, then |AP| = 3 and |PB| = 1. So the ratio of the two line segments is 3:1, and P is a point that is external to AB. Is this anywhere close to what your book is talking about?