Hi - can you clarify what you want to measure the frequency
of? I am guessing you mean sound, but want to be sure.
In general, you are correct that the unit of frequency is hertz (Hz), which is the number of repetitions of
something per second. But that "something" could be pretty much anything. It is commonly used to measure the number of complete periods of a sound wave and expressed in units of Hz. But for example, you can also use it to measure the frequency of electromagnetic radiation (light), or something physical like a rotating wheel - usually measured in rotations per minute (RPM), but can also be measured in hertz.
Assuming you are referring to sound, there are a variety of apps available that will do this with the microphone built into a smartphone. Search for things like "audio analyzer," "audio spectrum," "spectrum analyzer," etc. The apps will give you a graph where the x-axis is frequency and the y axis is some measure of the loudness* of sound at that frequency:
https://www.google.com/search?sca_esv=9 ... =911&dpr=1
* the units used to measure sound can be complicated, that is a story for another post.
Edit - I just realized that your profile says "other adult." This forum is intended for K-12 students doing science projects. I decided to reply anyway since students search the forums and this is a question that a student could potentially have when doing a science project.