A soft fork is a change made to cryptocurrency technology creating a temporary split in the group of recordings (blockchain). This change creates all-new, valid recordings (blocks) that are slightly different from the original blocks. They are just different enough that users of the new technology see blocks from original technology as invalid. But, users of the original technology see no problem with either one.
As a result, this means, new blocks will work just fine for all computers including those using the original technology. But computers using the original technology will find their blocks are rejected by the rest of the network until they upgrade and rejoin the network. Any new fork in the blockchain can fail and if it does, all users will return to the original recording.
Source: Decryptionairy
