A generic definition we would see over the internet would be: Frequency is defined as the number of waves that passes a fixed point for a given amount of time. But what does that really mean.

Variables on which frequency mainly depends on is wavelength and time.

Consider, the number of waves that could travel a point in one second. So, higher the wavelength lower the frequency and vice-versa.

In the above example, we can see a wave signal, with a shorter wavelength takes much lesser time to travel the same distance than a longer wavelength. The unit we measure frequency, is Hertz.

Here is how we could define Hertz based on a Waveform, w.r.t. time.

So, for a frequency of 2.4 GHz, the wave cycles oscillates for 2.4 x 10^{9} = 2400000000 per second and for 5 GHz wave cycles oscillates for 5 x 10^{9} = 5000000000

This should give an overall idea of what the frequency really means. It would be more interesting to understand how we channelize these frequencies.