Stem function and it's labeling.

Let I have a complex signal h, I take a=fft(h). Now if I plot b=stem(a), what stem plot x and y axis defines? . whats is the difference between plotting stem(a) and stem(abs(a)) though stem plot doesnt plot imaginary values?

[Daniel M]

doc fft

Also look at the examples <https://www.mathworks.com/help/matlab/examples.html?category=fourier-analysis-and-filtering>