Definition of Point
A point is an important concept in geometry.
You can think of a point as the mark of a very very sharp pen on a piece of paper.
Keep in mind that even a very sharp pen will make a mark that is not really 0 in size. A point has no size so pen marks are "bigger" then points but are a good mental representation.
A, B, C and D are four points. They are represented as little circles so you can be able to drag and drop them with the mouse. The points are actually the centers of the circles.
Note: Usually in formal geometry the point and the line and not really defined. They have only some proprieties (axioms) that are associated with them.