Boons and banes are d6s that are rolled with your d20 to Attack and Challenge rolls. They stack and cancel each other, but you only ever take the highest value. Example:
Say you're a warrior who makes attack rolls with weapons with two boons. So with a basic attack roll, you're rolling 1d20 + 2 boons. Ignoring attribute bonus for a moment, if you rolled (d20 = 10, d6 = 2, d6 = 4), you'd take the highest d6 (4) and add it to your d20 roll (total 14). Banes and boons cancel each other out on a 1-to-1 basis, so you don't have the weird situation that comes up a lot in D&D 5E where once you have one source of advantage, any further advantage is useless. And since the swing isn't as large, any situational modifiers can be handled by the GM adding boons or banes instead of flat bonuses or worthless additional advantage.
Weapon attacks can be modified by adding banes to your attack roll (lunging attack, just add a bane to the roll), which means that any character can attempt different types of martial attacks, but martially focused characters will have the spare boons available to still have a strong chance of success.
Also, while technically there is no strictly social attribute, the social stuff is handled between Intelligence (bluffing, logical appeals) and Willpower (charm, emotional appeals), and any relevant professions you happen to have (is your character a baker and talking to an NPC baker? That's probably worth a boon to the roll).
Sorry if that reads a bit confusing, I promise it feels very simple in play.
Edit: Also, as previously noted, SotDL is Horror Fantasy, not Heroic Fantasy. While it can do heroic fantasy ala D&D 5E, that's not its designed purpose. Which I can only assume is partially the reason for the flatter progression.
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