The angel banishing sigil has made for some pretty cool scenes in the show. What happens is, the sigil is drawn in fresh blood (and yes, it must be blood) and when it’s hit, it banishes all angels from the area. This often comes in useful for the Winchesters, who
sometimes need a quick head start on the angels hunting them.
It should be noted that angels too can use this spell, and aren’t banished if they’re the one using the sigil. For example, Castiel actually draws the sigil on his own chest at one point to banish angels, which is a really brave move. That had to hurt.