Ability scores can give small bonuses to your dice rolls. Universal d20-based system of roll-over d&d 3.5 eberron explorer’s handbook pdf numbers. EXP’s to advance to the next level.
Classes get features as they reach new class levels. Hit points, class-based hit dice. If a character has an ‘advantage’ for a skill roll or combat roll, the player rolls two d20 and takes the better one. If a character has a ‘disadvantage’, roll two d20 and take the worse one. Attributes are the same ol’ six, but more important than before. They’re used for skills checks and saving throws.