The rent cap (Mietendeckel) was a rent control law in Berlin. It set rent limits in each area, and it stopped all rent increases for 5 years. If a landlord charged too much rent, they could get a big fine.
It came in effect on January 30 2020, but it was repealed in court on April 15 2021.
The Mietendeckel is not the same as the Mietpreisbremse, another rent control law. The Mietpreisbremse is still in effect.