To add to delhanty's reply, a "degree of freedom" can be thought of as a dimension of the drawing that can be changed or "stretched" or moved without violating a constraint (this is slightly inaccurate, but it's a good start). In a CAD program, a fully constrained drawing can't be freely stretched or dragged around; the program won't let you and the drawing will feel "rigid".

It's very intuitive if you play around with a CAD program for a bit. There is a free (GPLv3) 2D and 3D CAD program called Solvespace (https://solvespace.com/) that is probably easiest one to obtain and learn. There are detailed tutorials on the website, and you could probably download it and finish the first tutorial in an hour.