Can someone please tell me what mobile data is in the pulldown notification bar & if I need it switched on? Thanks alot

Mobile Data is what you need to switch on when away from a Wifi network, if you want the Internet.
Make sure you have a data package with your network provider, and know how much data you can use.
I'm with giffgaff and on a £10 unlimited data package, so just leave my mobile data on all the time. It switches automatically to wifi when in range.