It states in the manual that it is suitable for ethylene glycol-based antifreeze and depending on the specific gravity of the sample/solution (ethylene glycol - deionized water) you will test, it shows you the freezing temperature of the solution, in other words, the antifreeze protection you have.
It consists of a tube that contains 6 plastic discs, each with a different weight. Depending on the concentration of the antifreeze - deionized water solution you add, a different number of discs will float. Theoretically, of course.

I tried it with GM's Dex-cool, the original coolant used by Opel, which comes concentrated and needs to be diluted with deionized water in a 1:1 ratio to have antifreeze protection at -38°C as specified by the manufacturer. (Of course, what's the use of antifreeze protection at -38°C in Athens is another matter).
I mixed the coolant and deionized water with a measuring device, so I knew I had the correct ratio of 1:1. Normally, 5 discs should float. In my case, all 6 discs floated, indicating that the solution has a high concentration of ethylene glycol. I diluted it to 1:1.25 (roughly 100ml Dex-cool and 125ml deionized water, meaning 44% ethylene glycol content) and then only 2 discs floated instead of the expected 3 discs.
Of course, Dex-cool (like most coolants) does not only contain ethylene glycol, but also other compounds that may affect the specific gravity of the coolant.
Overall, it doesn't seem reliable to me and I cannot rely on it to draw safe conclusions about the density of the coolant in the cooling system.