Mini storage/self storage is a room you rent for storing your things. E.g. if you are going to live out of the country for some time, maybe you would need a place to store some furniture, bikes etc. So you rent a room, typically it is just a concrete box you get a key to and you can put your stuff there. 

You can think of it as a hotel for your stuff. You would usually pay per cubic foot/meter rather than per square foot/meter.

Example http://www.brightonselfstorage.com/sizes-and-prices.php

They can also be low rent apartments, store your stuff and your family.

Thank you very much guys, I have found the mini storage, if  somebody need, I can introduce to you.

This is their website :http://koalaministorage.com/