What is Wave Frequency ?
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.
1 Hertz = 1 Wave cycle / sec, 2 Hertz = 2 Wave cycles / sec or 4 Hertz = 4 Wave cycles / sec

1 Hertz (Hz) = 1 Wave cycle / sec
1 KiloHertz (KHz) = 103 Wave cycle / sec
1 MegaHertz (MHz) = 106 Wave cycle / sec
1 GigaHertz (GHz) = 109 Wave cycle / sec
So, for a frequency of 2.4 GHz, the wave cycles oscillates for 2.4 x 109 = 2400000000 per second and for 5 GHz wave cycles oscillates for 5 x 109 = 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.
References:
http://www.techplayon.com/]
CWNA Study Guide