Slayer is an attribute that some units have, and can gain through various means.
When a unit with Slayer attacks it’s prey, it gains +5 to it’s attack. In the event of ranged attacks, the attack deals an additional 10 damage, after defense is calculated, thus guaranteeing a minimum damage of 10.
It is not possible to “stack” Slayer. If a unit somehow gets the same type of slayer twice, it still only counts once.