The 'plate solve' is software that analyses the stars in an image, and works out where you are - ie the RA/Dec of centre of frame.
The software can use this to tell the mount where to go....
With SGPro - I manually typed in the RA/Dec of M83, and told it to goto it - it slewed the mount to this area, took an image, plate solved that image, then issued a correction slew to the mount (as the initial slew was off) - then it takes another image, plate solves that, and issues further corrections until you are happy with where you are pointing.
You can also give the program your image from a previous session, it can plate solve that one, then line up your scope exactly on that spot again....
It's VERY cool....
have a play by uploading images (or crops from images) to nova.astrometry.net - you can see the info you get....