This adapter allows you to use old analog OM lenses on your m4/3 mount DSLR. It has a solid build and a firm fixing, so there is no risk of damage to the mount. There are a lot of unbranded versions of this product about, but that's one thing to be especially wary of; the last thing you want is a cheap bit of plastic ruining the mount of a camera you've just paid hundreds for!
It's also worth bearing in mind that gap between the camera and the lens created by this makes it a mild teleconverter - so a 50mm OM lens will result in being a little longer.
There are no electronics in this, it is essentially just a strong piece of metal, so autofocus won't work and it only works in Aperture mode or Manual mode. The aperture is adjusted on the lens, and with aperture the shutter speed will be calculated, or with manual you'll have to do it yourself. A massive advantage is that you can use the now quite cheap OM zooms - a 600mm OM lens is about 1/8th the price of a digital one. Also, in my opinion, combining the technicalities of manual lenses with the ease of a digital body makes a much better photographer.