A descriptive mark can be registered if the applicant demonstrates acquired distinctiveness (secondary meaning): that the mark, through long and exclusive use in commerce, has come to be recognized by consumers as identifying a single commercial source rather than merely describing the product.
In the US, the USPTO permits a presumption of acquired distinctiveness after five years of exclusive and continuous use, though additional supporting evidence; sales figures, advertising expenditure, consumer surveys; is typically required. The stronger the descriptiveness of the mark, the more evidence is needed.
Marks that cannot yet demonstrate acquired distinctiveness may be registered on the Supplemental Register (which provides some limited benefits but does not grant the same rights as the Principal Register) while the mark builds its commercial reputation.