Plot device
In Barthes's philosophy of art, plot devices are meaningful suggestions to the reader, in that they tell the reader about the reality of human life and nature. That means that the reader can learn more about life itself and about the order of the universe by reading. This theory would hold across all literature, potentially, but one example might be Infinite Jest by David Foster Wallace, where the plot is obscured. In Barthes theories, that might mean Wallace intended this suggestion about reality: "The patterns of life are obscured."
Symbol
A symbol is to a novel what a noun is to the sentence. A symbol is a crux of meaning where the reader is left to observe whatever can be known about the symbol. This is part of why simple symbols are better than mixing metaphors, because symbolism isn't telling the reader what to think, it is merely allowing the reader to jump to new associations in their thoughts about the work. One example in literature might be the littered shirts in The Great Gatsby, which might perhaps leave the reader thinking, "Who's going to clean all this up?"
Allegory
An allegory is what it is called when a narrative is symbolic, and often the attempt to liken reality to an allegory is implied. But, a reader will know the allegorical quality of a novel or story simply by understanding the way they sympathize with the text. When a tragedy evokes pity and fear, Barthes would say this is evidence of mythological allegory, which suggests that one's own life could follow the same pattern as the hero's tragic downfall.
Motif
A motif is more than a symbol being repeated a few times. If a writer mentions nature, even in a casual way, they are invoking a motif. If they continue writing about nature, then that motif grows, so a novel's major motifs will be the domains of meaning that are constructed by repeating associations. This is the same as saying "Motif is a map of meaning." If a novel mentions earth, wind, and fire in the prose, one might say there is an elemental motif.
The paradox of connotation and denotation
All of these meaning maps allow the reader to understand the way meaning is attached not just to bodies of literary work but also to words themselves which at their most basic and fundamental still carry these two hypothetical opposites: Connotation and denotation. In Barthes's analysis, the connotation of a word is the codified suggestion to the reader, and the denotation is the obvious sense of the word without referencing social constructs. So Barthes says the meaning is in the connotation and denotation, because there are common and independent types of meaning.
