MiFi stands for Mobile Wifi. A MiFi is a mobile router that, equipped with a SIM card, makes contact with a mobile internet provider. In this way you create your own "hotspot".
A MiFi is indispensable for anyone who ever uses mobile internet in trains, cars or boats or stays abroad. If you buy a local data SIM abroad, you can save considerably on roaming costs. Due to its ease of use, compact size and convenient functionality, the MiFi has become very popular in a very short time.