Coined by French author and critic Roland Barthes, semiotics is the study of the language of signs - and by signs, he means nonverbal clues.
In all nonverbal media (the graphic arts, theatre, film and television), semiotics is restricted to the use of visual clues that give you essential information, typically about character, setting or location. Barthes gives the example from professional wrestling, of how to recognise the good guy and the bad guy:
The good guy wears a mask that covers his upper face and (sometimes) his hair. The bad guy wears a mask that covers his whole face and head. Colour clues are also given - greens, blues, yellows, gold and white are used by good guys, combinations including black and red are used by the bad guys. (I don't watch professional wrestling but I'd be interested to learn if these signs still exist).
In film, he gives the example of how to recognise an ancient Roman (and distinguish him from and ancient Greek). Both Romans and Greeks in film wear togas, but Romans Have Fringes. This sign is so strong that even in recent years, film-makers have a hard time breaking free of it.
In verbal media, signs go under another name: symbolism.
Symbolism in literature is the use of visual clues, in the same way as in other media, however in literature it can go much further. From using an animal emblem for a character, through using repeated allusions to rotation, circles and wheels throughout your text to fix the idea of the Wheel of Fortune, semiotics in literature is rich and almost infinite in potential.
Arguably, the use of poetic imagery in literature is a subtle form of semiotics, as are analogy, simile and metaphor.
The author should therefore have a heightened awareness of words that have visual connotations. I just came across a borderline usage of "thundrous*" for a sound. Thundrous is normally a rolling, rumbling sound, but is often (carelessly IMO) used to mean "very loud".
'The thundrous beating of the rain on the tin roof' is one of those weird ones where while it isn't wrong, by associating it with rain, you activate the visual component - thunder makes you think of the sky when it thunders, of the sudden arrival of darkness, and clouds, dark with rain, heavy and pendulous... easy tiger. This might be what the author intended but in this case I didn't think so.
* I prefer the often supposed incorrect alternative spelling "thundrous". I think it should be encouraged. Your spell checker will tell you it is thunderous. Along with murdrous and wondrous.