Original: Beginning on 20 November 1923, 1,000,000,000,000 old Marks were exchanged for 1 Rentenmark so that 4.2 Rentenmarks were worth 1 US dollar, exactly the same rate the Mark had in 1914.
Simplified: The Rentenmark was introduced to stop this problem. The exchange rate was set at 4.2 Rentenmarks to 1 US Dollar.