This is a propeller balancer that uses magnetic suspension for greater accuracy when balancing the prop. A prop balancer is an essential piece of equipment for multicopter pilots as it allows you to fine-tune your props for super efficient flying. Out of balance props cause vibrations and noise which can lead to parts vibrating loose with time. A more noticeable effect would be in any video jelly effect/photos that are blurred due to unbalanced propellers.
Please note that due to manufacturing tolerances you might need to balance the actual blue aluminium tapered nuts that hold your propeller on the balance as they may not be perfectly balanced. This can be easily done with a small metal file. It’s suggested that you perform this step before trying to balance your propellers. The balancer comes disassembled.