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. 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. Differences are what makes life interesting!
As a rhetorical device, writers use synesthesia when they describe one sense in terms of another. It makes descriptions more interesting. For example, when a voice is be described as "sweet as honey" we are using one sense (taste) to describe another (hearing). When someone describes a "loud shirt," we don't picture a talking shirt, but one that has a vibrant, colorful pattern.
In my book, the Boy with 17 Senses, I tried to include many examples of synesthesia.