Semiotics - Signifier\signified
Signifier<>signified relationship
Same sound image - different concepts?
Some words do have meanings which are relatively easy to conceptualise, but we certainly do not have neat visual images corresponding to every word we say. Nor is there any guarantee that a concept which might come to mind when I use the word table is going to be the same as the one you, the reader, might bring to mind.
While that's quite correct, the fact remains that it also explains why Saussure's ideas took things forward. His notion of the sign places the emphasis on our individual 'concepts' corresponding to the sound images. Your mental picture of a car (indeed, for all I know, not only a mental picture, but also a mental smell, mental noise or whatever) will not be the same as mine, for a variety of reasons. (For a discussion of some of those reasons, see the section on Meaning).
Saussure shifted the emphasis from the notion that there is some kind of 'real world' out there to which we all refer in words which mean the same to all of us. Fairly obviously, we in our language community have much of this real world in common, otherwise we couldn't communicate, but, for various reasons, the 'real world' which we articulate through our signs will be different for every one of us. (It is for this reason that Saussure saw semiology as a branch of social psychology.)