This will generate a parameter called lfo that varies sinusoidally with an amplitude of amp and a frequency of freq. If you double-click on the SU transform in the editor, the three parameters freq, amp and lfo will be shown in a new tab. You can now set freq and amp and watch lfo modulate. The parameter names lfo, freq and amp are completely arbitrary. You can change lfo to be the name of a parameter in another VST and then connect the parameter output (green) of the SU transform to the parameter input of another VST, which will then have this particular parameter enslaved to your LFO.