What is 'Waqf by user' and why did Supreme Court raise concerns over this?

Supreme Court, while hearing petitions against Waqf (Amendment) Act, ordered Centre to not de-notify 'Waqf by user' properties till May 5, failing which there can be "grave ramifications". Properties are declared 'Waqf by user' if they don't have formal deeds or registration but have been in continuous use for religious or charitable purposes. The concept was legally recognised in 2013.