The "meaning" behind the slogan: Black Lives Matter

I've had confrontation after confrontation over the Black Lives Matter slogan, and it still makes no sense to me. I call it a slogan because it's being marketed - no really - people are actually making money off of this slogan, and yet there is no identifiable meaning to it. I've brought up the argument that "not only black lives matter" and I've been told that is not what this slogan means - "that's not what they are saying". Sorry, but that is exactly what is being said and how it is being marketed. Then I argued that: Ok then....so All lives Matter? Now that phrase was met with anger and hostility - so why?

Can anyone explain what this bullshit actually means or what it's all about?