Reebok is a sports brand. People say Reeboks especially for the shoes — trainers, running shoes, gym shoes — though the company also makes clothes and other kit. It is a proper name, so it takes a capital R. In English you might hear ‘a pair of Reeboks’ the way people say ‘a pair of Nikes’. It is ordinary commercial English: not formal, not slang. The name comes from an African antelope, the rhebok (sometimes spelled reebok), though almost nobody buying the shoes is thinking of the animal.