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As Leo describes, a Null interface is one, when sent packets, acts as a "black hole" (or bit bucket). I.e. they go in but they do go out.
One use of this, instead of using an ACL to drop packet to some destination, you can route those packet to a Null interface. The reason for doing this, depending on platform, is it might be more efficient than using an ACL. It could, I believe, than also be use to monitor bandwidth of such packets being dropped.
Another common use of a Null interface, is to create a prefix, via a static route, to be advertised by BGP.
