Frequency is very important parameter of any signal. No matter if you are generating or synthesizing signals, you most likely need to measure its frequency. When signal frequency is bellow 40kHz and amplitude is near 5V then you can build frequency counter on Arduino with standard LCD screen. If measure signal is 5V (or 3.3V) level then you can feed it directly in to Arduino input. In other cases you need to use limiting circuit like amplifier or voltage divider. But this small project assumes that voltage is fed directly to microcontroller input.
In this example signal comes to digital pin 12 where function pulseIn() counts number of pulses during specified time. Program itself is only few lines of code, and hardware is straight forward so putting it to work condition is only a matter of minutes.