Strength:

Your character's strength is, well, how strong he is. It's exactly what it says on the tin. Strength is used to determine carrying capacity, damage dealt in melee, the size of weapon you can wield, etc.. Strength is your muscles, plain and simple. Players who anticipate a lot of melee combat and heavy lifting should take a high strength score. The strength ability is independent of size, so a giant human that is only proportionally as strong as an average human has strength 3, despite the fact that he could easily lift cars. See Scale in Chapter 3: Combat for more information on size.