It's basically an invisible brush that is there for the only purpose of blocking actors from moving into a space. These can be used for complex objects where you may want to make them nonsolid, but still block movement (for map efficiency reasons), or if there's a space that a player could become trapped in so you do what's best for them and keep them out of it (unless you decide to make said space a killzone, always good for a couple laughs ).
The settings for what the BV blocks are under Actor/Collision.
Highlander: Re:Blocking Projectiles. Try setting bBlockZeroExtendTraces or bBlockNonZeroExtentTraces to true or fiddle with the settings under collision. (Not at home system so cant check)