Mi fi stands for mobile wi fi. The one I got is on 3 network.
Basically it 's a small dongle about the size of a small mobile phone that generates a wi fi signal.
You can get it with a pay as you go or contract sim.
We got a contact one, on a rolling month for £15 for 5gb of data. Was going to convert it to pay as you go as wasn't using it that much, but they dropped the monthly rate so kept it on.
It works just the same as home wi fi, except you have it anywhere. We use it at home often when our Bt broadband plays up ( about 3 times a week at the mo- grr!) Also use in coffee shops or the car when waiting for dc at activities.
Battery life isn't fantastic, but easily lasts a couple of hours and can be charged on a laptop or Pc, not iPad tho' cos it needs USB