Meaning of signifier and signified explained simply

According to de Saussure, the linguistic sign unites, “not a name and a thing, but a concept and an acoustic image”. The acoustic image is called the signifier : it is not the material sound, but the psychic imprint of this sound. the concept, called signified , contains the distinctive features that characterize this sign in relation to the features of other signs of the language. The linguistic sign is therefore defined as a two-sided psychic entity: signifier / signified. For example, the French word tree (arbre) is a linguistic sign associating the sound of "arbre"(signifier) with the meaning the type of plant which is a tree (signified) 

Saussure distinguishes four characteristics of his linguistic sign theory:

The arbitrariness of the sign: the link between the signifier and the signified is arbitrary, because the same concept can be associated with different acoustic images depending on the language .

The linear character of the signifier  : “the signifier, being auditory in nature, unfolds in time”. The elements of the signifiers are therefore necessarily presented one after the other, in a linear succession: they form a  successive chain.

The synchronic  immutability of the sign  : the signifier associated with a given concept is imposed on the linguistic community: a speaker cannot decide to modify it arbitrarily.

The diachronic  mutability of the sign: linguistic signs can nevertheless be modified by time, by linguistic evolution ( History of phonetics and modification of the signifier, of the signified or of their relation).