Soft forking means that after a rule change, nodes that have deployed the new rule can still interact with nodes that have not deployed it.
Take the block size as an example: assume the original block size is 2MB; from now on, half of the network nodes implement the new limit of 1MB block size and consider oversized blocks as invalid.
The old version of nodes can still receive or broadcast blocks.
In this case, the new network can cover all nodes without being limited by the rule version.
Bitcoin's Segregated Witness (also known as "SegWit") is one example of a soft fork. The fork cleverly introduces a new format for blocks and transactions. Nodes that are not updated can continue to receive block information, but do not participate in the validation of new transactions.