OEMs or Original Equipment Manufacturers are manufactures that create goods/parts that would be used with parts from other OEMs to, together, make a finished product.
It is important to note that the term OEM can be used to describe both companies that make parts, and parts that are from OEMs.
A popular example of an OEM is Intel. Intel makes OEM processors/CPUs that they later sell to companies such as Dell, Asus, HP, Lenovo, Apple, etc that end up in completed PCs.
It is also important to know that while some OEMs only make parts for other companies, there are also companies that make both OEM parts as well as finished goods for consumers to buy directly.
OEMs are different from ODMs (Original Device Manufacturers) which make finished products that can, in theory, be sold directly to consumers, but instead, sell unbranded products to other companies, which put their own branding on it. This practice is known as white-labeling.