Muscle
This affects how much damage is inflicted in combat, and generally helps with tasks of brute force, such as pulling one's self up from a ledge, holding a door shut against intruders, lifting a 10 ton boulder above one's head, or crushing your opponents' skulls like little tin cans.
A strong character need not have giant, rippling muscles. One could have above-average strength thanks to genetic experimentation, cybernetic implants, or supernatural powers. A little girl with a Muscle of 8 is a perfectly acceptable and interesting character concept.
A character with a poor Muscle score is a wimp, barely able to carry a backpack, and always need others to open the pickle jar. A character with a good Muscle score is a powerhouse: capable of untold feats of strength including busting down doors, carrying comrades, and tossing enemies.
Some examples of muscular people include weightlifters, furniture movers, and Hercules.