The Canon EF-EOS M Adapter is the key to unlocking access to the vast array of Canon EF, EF-S, TS-E and MP-E lenses to the EOS M/EF-M compatible EOS cameras.
The Canon EF-EOS M Adapter mounts on an EF-M mount compatible camera (as a lens does) and allows a Canon EF, EF-S, TS-E or MP-E lens to be mounted on the other end. The Canon EF-EOS M Adapter is very solidly built, but it is a very simple device with no glass, one switch (the lens release) and electronics to allow the above-referenced lenses to fully-functionally perform on an EOS M camera body.
The adapter weighs 3.7 oz (105g) and measures 2.74 x 1.21” (69.5 x 30.7mm) without the additional .46" (12.9mm), 1.6 oz (45g) of the tripod adapter (included) attached.
The removable tripod adapter allows the camera balance to be shifted rearward when tripod-mounted (since the EOS M cameras weigh very little). Of course, the shift distance is not very big - and the size of the mount platform is quite small. A lens tripod ring is going to be a far better solution for tripod-mounting larger lenses - and no lens is too large for this adapter.
That (above) is the Canon EF 600mm f/4 L II IS Lens shown mounted to a Canon EOS M camera via the Canon EF-EOS M Adapter. You likely already figured out that camera and lens balance is very front-heavy with even modestly-sized lenses, such as the Canon EF 24-70mm f/2.8 L II USM Lens, mounted. Plan on using such a lens to control the camera as the tiny EOS M grip becomes inadequate with the front weight.
If you have a Canon EF-M mount camera and want to use lenses in addition to the EF-M type, the Canon EF-EOS M Adapter is the answer to the problem. While not a cheap accessory, the Canon EF-EOS M Adapter adds especially great value to a kit with compatible lenses already in it.