A soft fork is a software upgrade that is backwards compatible with older versions. This means that participants that did not upgrade to the new software will still be able to participate in validating and verifying transactions. It is much easier to implement a soft fork as only a majority of participants need to upgrade the software.
While Hard forks refer to a software upgrade that isn’t compatible with older versions. All participants must upgrade to the new software to continue participating and validating new transactions. Those who didn’t upgrade would be separated from the network and cannot validate the new transactions.