Hi Abhilash
Fast ring and slow ring in the insiders preview program controls how the updates are delivered for windows 10 Preview. Preview is a trial software and any changes that the programmers make to windows 10 is released to fast ring first. So if you are in fast ring you get it before slow ring but that also means if there are any problems with the OS then you will also get it before anyone else and it may not even have solutions.
On the other hand the slow ring insiders will get the same OS update and a bit more which includes bug fixes and solutions to all the problems faced by the fast ring insiders. So while the fast ring might get 3-4 updates the slow ring might just get 1 update in that entire time.