Is this a different thing from push?
I mean if i turn this off for say whatsapp, am I likely not to be told of messages being delivered or does oush or whatevere it was called, still work and let you know messages are arriving?
Yes, it's different. Background refresh actually launches and runs the app in the background, allowing it to "catchup". For example, if Gmail app got a push notification, it would run the GMail app, get the up-to-date inbox ready for when you actually open it etc.
Just a battery drain if you don't mind waiting 5 seconds when you open the app manually for it to refresh.