Signifier =>The word or symbol to we use to represent something Signified => That to which we are referring to [the word or sound of "Tree" (signifier) to the object or concept when we hear "Tree" (signified)] - The object or idea itself is the 'referent'.
There's no reason the letters C-A-T (or the sound the word makes) produce exactly the image of the small, domesticated animal with fur, four legs and a tail in our minds. It is a result of "convention".
One thing is defined simply as being different from another. Language is a "system of interdependent terms (whose value) results solely from the simultaneous presence of others" - so words mean nothing without the other words around them. You can't have a "stand-alone" element.