The TextCursor (not the MousePointer) defines the position of point, which is the text insertion position. Point is one end of the region. The other end is the mark. The mark is invisible (you can MakeMarkVisible).

Though point is spoken of as being the cursor position, this is really only true when the cursor is shown before or after a character. This is the case, for instance, for a bar cursor. When a block cursor is used, it is positioned on top of the character that follows point.

You can set point by clicking ‘mouse-1’ or using the arrow keys (‘up’, ‘down’, ‘left’, and ‘right’).