It occurs to me that I might want to do a "simple summary", so here it is:
Abilities
All creatures have the standard D&D Ability scores, which always range from 3 to 18, regardless of race/species. Each Ability score produces an Ability modifier, following B/X scaling (3 is -3, 4-5 is -2, 6-8 is -1, 9-12 is +0, 13-15 is +1, 16-17 is +2, 18 is +3).
A creature's race/species will provide a racial adjustment to one or more Ability modifiers; player character races always provide a +1 to one fixed Ability modifier and a +1 to another where the player can choose between two options (for example, Elves provide +1 to Wis and +1 to Dex or Cha).
Proficiencies
Characters, NPCs and monsters have a 'base combat proficiency', plus 18 skill proficiencies that they can level up. Each proficiency starts at +2/+d4, and increases by +1/+die size up to a maximum of +6/+d12 whenever a skill point is invested. Player characters get 1 skill point per class level (Expert classes get an additional +1 skill point every odd class level), and skills caps increase by +1 every 5 levels (or every time they take a feat or class feature that explicitly grants a skill point).
Combat proficiency is based on level (with Warrior classes starting at +3/+d6 and gaining +1 every 4 Warrior class levels, Experts starting at +d4 until class level 2 when they count as Warriors at half their Expert class level, and Mages starting at +d4 and staying there).
Hit Dice, Hit Points, Wounds, and Fatigue
Each class gets 1 hit die at level 1, and +1 hit die every 4 levels; these are used for rest-healing just like 5E hit dice. A character's hit dice determine their maximum skill proficiencies, as mentioned above.
A character's base hit points equals their Constitution score; they also gain hit points each class level equal to that class's hit die average + their Con modifier.
Characters also have Wounds, which are the equivalent of "failed death saving throws". You can gain Wounds by failing a death saving throw (obviously), but also by taking critical hits, or by explicitly life-draining attacks. A character dies when they've suffered more Wounds than their Con mod + 5.
Characters also have Fatigue, which works approximately like 5E's Exhaustion. Wounds and Fatigue both count towards Exhaustion, so when your total (Wounds+Fatigue) exceeds your (Con mod + 5), you're done for the day. Between (Con mod +1) and (Con mod +5), you start being Exhausted, which imparts conditions like Dazed or Slowed as your Exhaustion increases.
That's basically it; everything else is just details unique to a particular class/skill/monster/scenario.
