Mobile wifi

Hello,
I was looking to buy a JR pass from your website. Just before the confirmation step, I was given an option to purchase "Mobile wifi" for £55 for 14 days.

Please could you explain how this would work? I have a UK number which intend to take with me to Japan.

Questions:

1>Is this mobile wifi like an INTERNET dongle which I need to plug into a laptop/ wall socket?
2>What do I need to do to activate this mobile wifi on my UK phone?
3> Would you advice carrying a spare phone and buying a local sim on Japan for the duration of my stay?

Thanks for your help!