"OEM" refers to the Original Equipment Manufacturer. It means that the keys were originally sold as part of a set with some other equipment. In the end, however, the keys were not necessary for it and therefore were put separately on further sale. That is why the price is lower than in retail.

Windows OEM keys can be activated only once on a motherboard and it is locked to that device. Any hardware change or PC formatting might deactivate the license. Please keep in mind that when formatting your PC while the OEM key is activated, deactivate the key and it will not be usable again. 
Additionally, the OEM license cannot be used to upgrade or downgrade an operating system - you have to perform a clean installation before.

Also, it might require a "phone activation". More information on this subject can be found under this link.