When you attempt to fold a piece of paper, in particular a sheet of cover paper, the resulting edge of the fold will be cracked and may have an uneven fold. To help minimize/prevent the cracked edge and uneven fold a score can be done where it is to be folded. A score can be done a couple of ways.
The 1st way is a litho score which is done on press during the printing process. A rule is attached to the cylinder of the press creating the score. A litho score can not be used if the score needs to go on the wet printing area.
The 2nd way is with a die score. This is done after the piece is printed and done using a steel rule. It creates a deeper score and will work better on heavier stocks. A die score can be done with different thickness to create different size channels. Whenever printing on a cover stock, especially one with heavy ink coverage, it is worth the expense to score the piece.