The idea of the quick change tool post (QCTP) is exactly as the name implies. A good unit will allow you to quickly change tools as you change operations or need to replace an insert in a holder.
Basically you take a tool, whether an HSS bit, carbide with insert, boring bar, cutoff blade etc. Put it in a holder. place holder in machine, set height. Lock the height adjustment. Now, every time you use that tool it will be exactly where you left it set.
Need to swap out an insert? No worry. Remove the holder, swap insert, put back in the lathe. It will be exactly where you set it previously.
If you've ever used a lantern style or turret type tool post you'll see just how much time is saved with a QCTP. A godsend.
Most QCTP setups require only the rotation of a handle to either (or both) swap a tool holder or reposition the post. Again, big time savers.
Occasional maker of swarf.