see also: Fork. A Soft Fork is a small change in a crypto protocol. The current rules will be aggravated by adding new rules. The old rules are still valid. Advantage: Not all users needs to update the new protocol. A Soft Fork is also downwards compatible.
Old nodes, which are not updated yet with the new rules, understand and accept the changed protocol only if a majority of 51% update their software.
As long as there is no majority, a Soft Fork Blockchain is still running parallel to the old blockchain. With the Soft Fork a temporary split in the blockchain is created.
Consensus reached: YES
If a consensus is reached and the majority of 51% has confirmed the new rules, the new blockchain will substitute the old blockchain and will be understood by all participants in the network. Therefore a Soft Fork is downwards compatible.
Consensus reached: NO
If no majority is possible, the new parallel blockchain, which is created by the soft fork, will expire. The original blockchain will be continued unmodified.
Soft Forks are usually used for Blockchain Updates.