Hello Iftikhar, 

Centralised admission means that you can only fill out the course you want but not ask for any specific college. For example you may choose the BA course in your form, but you cannot choose any specific college to get enrolled into,  you can only enter your top college choices. You can be assigned to your chosen course in any college under Punjab University on the basis of merit. And non centralised admission is the normal form of admission where you choose your own college and apply there. 

Hope this helps :)