As wood dries out, it tends to twist and turn in all kinds of directions.
Generally furniture grade lumber arrives in a shop with a bit of a bow along its length, a bit of a cup across its width and maybe a twist to boot. How do you get this rough cut twisted lumber into perfectly straight and flat boards with which one can make decent furniture joints?
Some people think that a thickness planner is all that is required, but no -- there are three machines necessary to 'dress' a board.
SAW
First cut a long board to just a few inches longer than is necessary for the job, usually cutting off any split ends. This will lessen the total curve of a bow and reduce the amount of wood that must be removed to make a flat board.
JOINER
Then place the cup side down on a 'jointer'. This is a special cutting machine with an in-feed and an out-feed table at two different heights. Each time you run the board through it will remove exactly the difference between the two tables. It is a bit of an art to learn just how to use a jointer to obtain a perfectly flat surface with the least removal of wood possible, but it is this machine that can create a flat surface with no reference point. You cannot dress any board wider than the blade on your jointer. Once this surface is made, everything else is cut relative to this surface.
The next step is to place the flat surface against the fence of the same jointer which is usually set at exactly 90 degrees to the table. Now you remove the imperfections in the edge and end up with one face and one edge straight and at 90 degrees to each other.
TABLE SAW
Then we move over to the table saw where the face sits flat on the table and the straight edge against the fence. The saw blade will make the other edge exactly parallel to the first edge. Sometimes we return to the jointer just to make one light pass on this cut edge to remove the marks from the circular saw blade. Now we have one face and two edges dressed.
THICKNESS PLANER
The third tool is a 'thickness planner' that cuts the material off of the top face of the board, exactly parallel to the first flat face and at any specific thickness.
Le voila -- a perfectly flat and square board ready to become part of a piece of furniture.
Why bother with all of this when you can buy dressed wood at the lumber yard? Because that dressed wood at the lumber yard has probably picked up some more bow and cup while sitting on the shelf. When you do it yourself, it is ready to go, and as well, you can usually get slightly thicker boards because in the factory they pump it all out to a minimum standard.