When a blockchain is softforked, it means not all nodes upgrade to obey new consensus rules but only majority of miners are required to enforce the new rules. When a soft fork happens to a blockchain, transactions or blocks that are valid in the past will no longer be valid. So in a soft fork event, newer blocks are only seen by old nodes to be valid which makes it backward compatible.
Soft forks can be temporary if miners upgrade to new consensus rules unknowingly and need to restore the blockchain protocols arises. New functionalities can also be added on any blockchain utilising soft forks as this has been done on bitcoin and ethereum blockchains in the past.