Tab is like an interest free loan. So, if you want to upgrade your phone and want to pay less upfront, you can put up to $360 on the tab. Then you pay monthly tab charge next 24 minth to pay down your tab.
Say if you have $30 plan and want to to buy a $360 phone. You pay $0 upfront and put $360 on the tab.
Your monthly bill will be: $30(plan) + $15(tab charge).
If you pay more upfront, it will be less monthly tab charge.