If all you care about is how much power is in the signal as a function of frequency, then you can supply a single array of values as the real part of the signal and supply an array of zeroes as the imaginary part of the input signal.
You can do the FFT, then combine the sine and cosine outputs into a single power spectrum array which will tell you the power as a function of frequency.
So if you don't care about the phase relations of the input signals then you don't need to supply any actual data for the imaginary input array.
A real FFT function, real because it use real numbers as input. May use an array of samples from an ADC as a input. The output from such a routine will be some sort of complex number. This complex number can be decomposed in a amplitude and phase spectrum.