sketching frequency domain representation
This is a classical example of beat. This means that the signal (assuming that it is periodic and extends to plus and minus infinity) contains only two frequencies with the same amplitude. The frequency of the sine wave which seems to enclose the signal will be equal to the difference between the two frequencies. The frequency of the sine wave which seems to be modulated by the enclosing wave will be the mean of the two frequencies which are present in the signal.