Select the first line or point. This line or point will keep its position.
Select the second line or point. This line or point will move so that it is coincident with the first line or point.
Two line segments are coincident when they are on the same line. Two points are coincident when their position is the same. A line segment and a point are coincident when the point is on the line.
Add more coincidence constraints by repeating steps 2 and 3 or finish adding coincidence constraints by pressing the ESC key or the middle mouse button.
Coincident.