Let's start with what we know. This is called a Flash Blue Side String (NSFW). It's made by a French undergarment company called Alter out of a "high elastic" content fabric. Here's what we don't know: Literally anything else that is going on here.
It seems, at a glance, that the Side String tubes your penis into a sartorial sausage skin, and fastens itself to your hip like a garter, only if garters looped up through your asscrack. It is functionally a thong, basically, except on just the one side. And if a thong plastered your dick to your lower abdomen as though you were perpetually called to read in front of the class in a forever middle school.
A few immediate concerns: Why? Won't that create loads of undue pressure on some or all parts of your package? Is it more or less convenient for going to the bathroom? What happens when you get an erection? Is it predicated on having an erection? Can this possibly be healthy for sperm? Why??? What if it's actually comfortable? Am I making fun of this because I don't understand it or because it's an elastic blue tube for my cock that I wrap around my leg? Assuming these ever become even slightly acceptable to wear, can we enact a federal law requiring full body shaving/waxing/not making everyone vomit? How would someone compensate for the inevitable razor burn with a strap tight enough to keep everything in place? Why.
Diving into a Google search for "side string" does not offer much in the way of explanation, as that's typically used to descripe any underwear with, well, strings along the sides, not dicks. Google images has turned up a few traumatizing and pubetastic examples of what his might look like in real life (NSFW). It is however bereft of any satisfying explanation of the product or half-assed Flea references.
We do know that Alter produces other modestly impractical underwear, such as a thong that dresses your junk up like a pouch of Akasha spirit crystals. And also that at least one possibly human user was happy with his (?) purchase, judging from the lone review on the site [in translation]:
Strng out of the ordinnaire but comfortable, a good maintenance of the male anatomy. Sexy and sensual desire. (Try it). 4/5 Stars
Beyond that, this is a mystery. We bought 30. [Inderwear]