As ssy3141 said, in the literal sense, it is a raised level surface. At a train station, it is the raised area above and beside the rails, which allows you to walk onto the train. At almost all train stations, these platforms are numbered, to allow you to find the right train. However, by analogy, some bus stations also call the places that the bus stops to pick up passengers a platform. In that case - the platform may not in fact be raised at all! It is not clear from your passage if it is a bus or a train...

it means a raised level surface on which people or things can stand.