the quarentine tank is somewhere you can house your new arrivals temporarily to observe for diseases/parasites. you would QT your new arrivals for about 4 weeks (depending on signs of disease or parasite) before introducing them to the DT.
this protects the new arrival but more importantly, it protects your DT inhabitants from catching any disease or parasite your new arrival may have...
it also serves as a great place to medicate fish that do have a disease or parasite...
the QT is a simple set-up
a tank (size depends on what you intend to quarantine 10-20gal)
a light
a heater/thermometer
a hiding place (piece of PVC)
to add beneficial bacteria to the QT you can soak a filter sponge in your established DT for 24-48 hours and put it in the QT
hope this helps