Think of a send as a "copy" of the original track.
The track OUTPUT will go either to sub-buss for grouping purposes e.g "Drums", "Guitars", "Vocals" etc or dorect to your master buss
The track SEND will copy the track output to another buss for parallel processing, such as "Reverb", "Delay", "Parallel Compression" etc where the affected signals are blended back in with the original.