scrub is a keyword which enables network packet normalization, causing fragmented packets to be assembled and removing ambiguity. Enabling scrub provides a measure of protection against certain kinds of attacks based on incorrect handling of packet fragments. A number of supplementing options are available, but we choose the simplest form which is suitable for most configurations.
scrub in all
Some services, such as NFS, require some specific fragment handling options. This is extensively documented in the PF user guide and man pages provide all the information you could need.