What is imaginary part after IFFT and how to use it?
And as far as I know the real numbers mean magnitudes of each frequency bin, and imaginary numbers mean phase shifh of each frequency bin.

No. The magnitude of the complex spectrum $|X(k)|$ means magnitude of each freqency bin, and the phase angle of the complex spectrum $\angle X(k)$ represents phase shift.

But then what mean imaginary part of IFFT output?

If the input of IFFT is conjugate symmetric, the output of IFFT must be a real signal, and vice versa, FFT of a real signal must be conjugate symmetric.