A soft fork narrows what counts as valid. Old nodes see the new blocks as legitimate because they satisfy the old rules too, so the network can upgrade gradually without splitting.
This backward compatibility is why most upgrades to conservative chains ship as soft forks. The risk is subtler: non-upgraded nodes are trusting rules they cannot fully check, and a soft fork adopted by only part of the hash-rate can still cause temporary instability.
Example: cutting the maximum block size from 2 MB to 1 MB is a soft fork, because a 1 MB block is valid under both rule sets. Raising it to 4 MB is a hard-fork, because old nodes would reject the larger block outright.
Related: hard-fork, full-node, consensus-mechanism, chain-split