A B2B company is a company that sells their products Business to Business. Meaning, their primary customers are business leaders rather than individual users of a product.
One example of a B2B company is Stryker, the medical device company that makes a variety of surgical tools. As a business themselves, Stryker sells their surgical equipment to other business, specifically hospitals.
Thus, the B2B sale in this case is Stryker - to - Hospital.