A magnetic cup holder is a device that attaches to a surface (such as a refrigerator or bar counter) using a magnet and holds cups, often using either magnetic attraction for metal or special cups, or a mechanism where the bottom cup is pulled out and the next one drops down.
This is convenient for the kitchen, office, or bar to keep cups handy, but in the context of beverages, magnets in holders do not harm smartphones. This is convenient for the kitchen, office, or bar to keep glasses handy, but in the context of beverages, the magnets in the holders do not harm smartphones. 
How it works:

    Magnetic base: Attaches to a metal surface.
    Holder: Either a magnetic cup holder itself or a mechanical dispenser with springs that “releases” the bottom cup when it is removed.

What it is used for:

Saving space in the kitchen or office.
Convenient access to disposable cups at coolers or coffee machines.