When discussing the nature of the linguist sign de Saussure criticizes the notion that things precede words. When relating to the lingual sign what de Saussure essentially does is to replace actual referential reality with the signified. What the signifier points to is not something which exists outside of language, but rather to a meaning which is contained within human consciousness. The division between signifier and signified, which together compose Saussure's lingual sign, is the basis for his subsequent proposition that everything gains it meaning out of being in structural oppositional relations with other components.
When discussing the nature of the linguist sign de Saussure makes his famous statement about to lingual sign being arbitrary. The arbitrariness of the lingual sing is easily demonstrated by pointing to the fact that different languages have different signs for the same denotations. But this points to another matter. Were words representations of preexisting concepts all languages will have parallel words. But we do know that different languages cover the world of meaning with differently divided semantic networks. This means that language does not simply describe reality, but is in fact something separate and autonomous from it. When de Saussure says that the lingual sign is arbitrary he means it not it the sense that anyone can make up words, quite the opposite, signs according to Saussure are all conventions that are socially constructed. The linguistic sign, in other words, is arbitrary but is not open for free choice; its meaning is imposed on us by our linguistic surrounding.
De Saussure's ideas regarding the nature of the linguistic signs were of huge influence in the 20th century and were the corner stone of both structuralism and semiotics. Saussure's revolution is in making language relational into itself, it is not fixed nor predetermined, and it was now up to philosophy, sociology, linguistics and other adjacent fields to examine the manner in which a signifier is tied to a signified.