Admittedly, Saussure is not very interested in how communities agree on fixing or changing the relationships between signifiers and signifieds. The easiest example of this is a sentence, where the words come one at a time and in a line, one after the other, and because of that they are all connected to each other. One of the questions philosophers have puzzled over for centuries is whether ideas can exist at all without language. In other words, from Saussure's point of view, thought cannot exist without language. Sound is no more fixed than thought, though sounds can be distinguished from each other, and hence associated with ideas.
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.