Everyone is different. Some people are strong, others charming. Some wise, some healthy. Ability Scores let you know where your character stands with their physical and mental capabilities. Ability scores can also impart bonuses or penalties to skill checks.
For every 2 points above or below 10 you get +1 or -1 respectively as a Modifier for that Ability and all related Skill Checks.
• 1-2 (-5 Modifier): As smart, dexterous, wise, and charismatic as a rock. As strong and healthy as a dead rat.
• 3-4 (-4 Modifier): As smart, wise, and charismatic as a really dumb Goblin. As strong, healthy, and dexterous as someone dying of the plague.
• 5-6 (-3 Modifier): As smart and wise as higher animals. As charismatic as an antisocial person. As strong as a child or as dexterous and healthy as an elderly person.
• 6-7 (-2 Modifier): Dumb, but not stupid. Foolish, but not a git. As charismatic as an imp, as dexterous as a clumsy person. As healthy as a sickly person.
• 8-9 (-1 Modifier): Just slightly below average. Definitely nothing special here.
• 10-11 (0 Modifier): Completely average. Probably nothing special here.
• 12-13 (+1 Modifier): Just slightly above average. Possibly something special here.
• 14-15 (+2 Modifier): As smart and wise as a competent mage. As strong, dexterous, and healthy as a trained solider. As charming as a local politician.
• 16-17 (+3 Modifier): As smart and wise as a powerful magi. As strong and healthy as a circus strongman. As charismatic as a member of a Senate and as dexterous as a circus performer.
• 18-19 (+4 Modifier): As smart and wise as an arch mage. As strong as an ork and as healthy as a physician. Can charm people with ease and as agile as a master gymnast.
• 20+ (+5 and on…): The abilities at this point and beyond are legendary. It is rare to ever see anyone this innately good at anything.