As a writer, I find synesthesia intriguing in part because it is both a neurological condition and a rhetorical device. The neurological condition is explained in the video below, and if you are unfamiliar with it, you should watch it because it's fascinating. It's also a great reminder that everyone's brain is different. I know from experience that many kids grow up hiding what makes them feel different. I'm glad this condition is getting more exposure because I don't want any child to feel like they are "weird." Differences are what makes life interesting!
As a rhetorical device, writers use synesthesia when they describe one sense in terms of another. For example, when a voice is be described as "sweet as honey" we are using one sense (taste) to describe another (hearing). Colors don't have a temperature, but we describe some as "cool," using an adjective normally assigned to physical touch to describe something we see. When someone describes a "loud shirt," we don't picture a talking shirt, but one that has a vibrant, colorful pattern.