Hi Ahmad, thanks for posting. I'm flattered that you want to begin your wargaming journey with DEAD GODS and I'm more than happy to clarify.
In this game, the higher the stat, the better. You have to roll equal to or under a stat to succeed. This means that a negative modifier like -1 will reduce that roll by 1, which is a good thing.
On the reverse having a +1 modifier to the roll isn't good because it increases your die roll. This is why the rules call this a penalty.
F- just means that character doesn't have the Firing stat, so they're unable to make ranged attacks.
For the Slayer special feature the B4+ means having a Battle of 4 or more. So a model with the Slayer ability is going to be more effective against models with high Battle.