A prescaler is a device that can divide a radio frequency signal a number of times, as needed. This division has the role of preconditioning the useful signal to be, for example, compatible with the input of a circuit or a measuring device / instrument: frequency meter, oscilloscope etc. Ex. if the maximum input frequency of a frequency meter is 100MHz and our measurement signal is in the order of 1GHz, we need a prescaler with a divide ratio of at least 1:10. With this prescaler, your frequency meter up to 50MHz now becomes able to measure up to 1.5GHz without any limitations.