The "Reverse Path" is an address provided by the sender of an email. If the Reverse Path Filter is specified, the reverse path must contain the specified value. Otherwise, the mail will bounce and the sender will receive the error "this recipient requires that the reverse path contains xxxxxxx" (where xxxxxxx is the specified value).
This field should normally be left blank.
However, if you start to receive spam to an alias, then the reverse path filter gives you an alternative to deleting the forward or setting it's REA to bounce, if you know that the sender should always contain a certain value.
For example, suppose you have given Bigdaddy domain services an alias, and this alias is somehow harvested by spammers. Rather than having to set up a new alias for Bigdaddy (and you've noticed you have to do this every few months), you could specify "bigdaddy" in the reverse path filter. This is effective because the reverse path on all email from BigDaddy contains "bigdaddy" in some form or other, and secondly, because it is not cost effective for a spammer to forge the reverse path to "bigdaddy" to spam only you.