The correct rule is you lose 1 HP
per attack. The quickstart rules are an older version. I'm hoping they'll be able to fix that, you might have gotten an older box set that had the old rules in it.
And the M+1 means Model +1, or whatever strength your model has +1
It's mostly for melee weapons that add some extra strength to your attack. Usually the correct number will be in parenthesis on the card, like M+1 (4).