TL;DR: alt-right guys pretended the was a white power symbol in order to try to ‘own’ liberals by making up something so ludicrous it would seem ridiculous to highlight it. But they forgot that white nationalists would also read their stuff and so it got adopted unironically.
Or possibly ironically but as we know you can’t really do that unless you’re actually performing something where your audience understands there is irony to your actions. Either way, it is now a White Power symbol.
So don’t go using it without knowing that.