The number of oscillations per second. Unit is Hertz (Hz).
Here you see a signal, which has five complete oscillations in a time interval of exactly one second, the frequency of that signal is therefore
f = 5 / 1sec = 5Hz.
This is the picture you would see if you look at this signal with an oscilloscope. An oscilloscope shows signals in time domain.
But you can describe the same signal in a second way. The same signal in the frequency domain looks somehow different. The signal an spectrum analyzer would show looks like this:
As you can see, this signal appears as an single line in the frequency domain. This is only true for ideal sinoid or cosinoid signals as in this example. If you have signals with any other shape (rectangular, triangle, modulated signals, ..) in the time domain you will see here more than one line.