Basically, a Soft Fork is a software upgrade that is backwards compatible whereas a Hard Fork isn't. This means that people who don't upgrade when a Hard Fork is implemented will be kicked off the network, while people who dont upgrade when a soft fork is released can stay on the network but they wont be able to see/use the features implemented in the Soft Fork. With Steem a Hard Fork was implemented because it would be messy if there were some users earning more than others. With the BIP 148 Soft Fork, once the initial chain-split resolves, users who havent activated SegWit can still remain on the network and detect new blocks. However, their transaction speeds will basically be slower. But for the chain-split to resolve, the majority of users must accept the Soft Fork. Therefore, only a minority of users will be using old software.