! The spectrogram of FM signal has been generated by using Matlab.Now I want to what does these scratch like lined represent, though I have only two frequency components in my message signal(https://i.stack.imgur.com/CIN23.jpg)
The spectrogram represents the strength of frequency components of a given signal as function of time. To extract frequency information of the signal at some time you will inevitably use a window of your signal that's centered at the time of interest and then perform a frequency transform. However, you need to be careful when doing this. The shorter this window is, the less precise the spectrogram is regarding the signals frequency content. Furthermore, it appears that you are directly performing Fourier transform on that window of time without pre-scaling it (more about windowing functions) which means that you are likely seeing artifacts due to what is called "rectangular windowing". If you want to visualize your signal better, I would recommend using longer time windows and prescale each sub-section of your original signal by a hanning windowing function before performing Fourier transform. MATLAB documentation provides some useful information about the procedure and gives helpful examples for you to try: (matlab spectrogram).