Sounds then serve as signifiers for the ideas which are their signifieds. This is important to Saussure because he wants to insist that language is not a thing, a substance, but a form, a structure, a system. The first is that dissimilar things can be compared and exchanged, and the second is that similar things can be compared and exchanged. A good example of this is money. A dime is a signifier connected to a signified of 10 cents of something.
Words work the same way.
One signifier has meaning within a system, not because it's connected to a particular signified, but because it is NOT any of the other signifiers in the system. You might think about the letters of the alphabet in this context. Another good example of this is the digital languages recognized by computers, which consist of two switch positions, off and on, or O and 1. O has meaning because it is not 1, and 1 has meaning because it is not 0. To find out what a word or sign is not, you compare it to some other word or sign.