this is on the exports manpage for you to see, please check manpages next time. until then though, i'll cut and paste what it says there.
originally written in 'man exports'
sync Reply to requests only after the changes have been committed to
stable storage (see async above).
In releases of nfs-utils upto and including 1.0.0, this option
was the default. In this and future releases, sync is the
default, and async must be explicit requested if needed. To
help make system adminstrators aware of this change, ’exportfs’
will issue a warning if neither sync nor async is specified.