These strobe disks are for checking/adjusting the speed on your turntable, assuming it doesn't already have this feature. When using the applicable disk, viewed under fluorescent or other 50 or 60hz light (neon or similar) with your turntable rotating at 33-1/3 or 45 rpm, the respective dark bars on the disk will appear to stand still,

As mentioned, there are two versions included -- one for 50 and one for 60hz. Each disk has markings on it to avoid confusion. BTW the 50hz version includes a best-approximation of 45rpm, it's impossible to get it perfect with an integral number of slits.