A Mifi hotspot is a brand name used for internet routers that are portable wifi hotspots. These are not phones, they are routers. Routers that can even fit in your pocket.
Multiple devices can be hooked up to this wifi hotspot that supports 3G and 4G LTE data speeds. This is different then your phone's hotspot capabilities. The amount of data and speeds a Mifi can produce, will outshine a phone's performance by a longshot.
If you are interested in purchasing one of these MiFi hotspots, you should contact your carrier to see if they have any in stock. Stores like Walmart, Target and Best Buy also carry them.
Now when it comes to the actual connection there are different data plans. Some carriers offer pay as yo go plans where others require a monthly fee or an annual contract. It all depends on your carrier.
Click here to see a WhistleOut guide on Tethering
Click here to see How to turn on wifi hotspot sharing on your phone