When we analyze this relationship, we run into the issue of differentiating value & signification. When I first looked at the two words, it was easy to consider them the same, but they're not. Signification is a pretty solid and non-conformed meaning. For value of a word, it can be exchanged and even compared. The value of a word is "determined by its environment". We go back to Saussure's sheep example. The French word "mouton" and the English word "sheep" are compared but hold different values. The word and concept of sheep is represented and has the same significance in both languages, but in English "sheep" can also mean other things and be represented with other words. In French this isn't the case. This is the same for several words or phrases. Two or more languages can have the same signifier, but not all languages are able to express it through words. For example if you tell a Spanish speaking person and an English speaking person to describe how their weekend was and they both attended a party, they both know the experience and the concrete visuals and sounds of "having a good time at a party", but one may not have the particular words within their language to describe that experience.
When tying this back to Post-Structuralism, the relationship between the signifier and the signified is still arbitrary but this time it is considered stable. My personal understanding of this is that they are mostly going against structuralists' definition of difference: "we know what something is by what it is not". Our definition is stable because we accept the fact that there is more than one meaning within a text and that there is more than one overall distinction or "difference". This concept will not change and that is what makes it stable.
Hopefully I haven't further confused anyone. Just my gist of the lesson. Thanks for reading :)