It's a replacement for the magazine release on a rifle that makes it so you need a tool (like a bullet tip, hence the name) to push and release the magazine. CA law bans certain configurations with a detachable magazine. I always thought it was stupid to go the Bullet Button (it's actually a brand name) route because it limited you to ten round magazines that couldn't be changed easily. Fine for toys, but not for tools. My AK was originally a "featureless" gun - no pistol grip (MonsterMan Grip instead - Google it) and nothing else that would put it into "assault weapon" status. I got to use 30 round magazines that way, too.
That's all good if you had "grandfathered high capacity mags" but for those of us that didn't live there when the ban took place we had to do what we had to do. Not bashing in the slightest, I seriously debated going the monster man grip but knew I was gonna be leaving the state and wasn't planning on keeping it that way.