An album "goes platinum" once it has hit a certain number of sales. The exact number of album sales required to go platinum varies from country to country, depending on population. In the US, it takes one million sales for an album to go platinum, while in Slovakia, 6,000 sales equals a platinum record.
The term "platinum" was devised by the RIAA. There is a national recording industry agency in most countries that is responsible for certifying sales and awarding platinum status to albums in their territory.