Well, they aren't grass. One of the several reasons Chinese gardens can seem so alien is that there is very little of the greensward that serves as the 'floor' of most of our landscapes. In Suzhou, it is only the Humble Administrator's Garden, the largest of those open to the public, that has any expanse of lawn, and that is perhaps why it is also the one that feels most familiar, most comprehensible, to at least this westerner.
I don't think it's that the Chinese don't like the lawn, and certainly a grass floor fits well with their conceptions of the natural landscape; it's simply that rocks are far more important. And in a small garden, where there is not perhaps room for both, rocks definitely win the day.
Within a single garden, there may be twenty or so different arrangements of small rocks and pebbles and roof tiles set on edge, mortared into place in decorative patterns. They are more than paths, though they are used as such--spreading out across courtyards and walkways alike to add a subtly changing carpet-like quality to the spaces.
These garden floors are one of my 'take-home' ideas from China.