Attack

A character's Attack represents their accuracy with a weapon, be it melee or ranged. An attack roll is made each time a character wishes to hit a target with a weapon. When a character makes their attack roll, they roll a d20 and add their attack bonus (other modifiers may also apply). If the attack roll is equal to or greater than the target's AC, the attack is succesful and the character may then roll for damage.

A natural 1 (d20 lands with the 1 up) is an automatic miss whereas a natural 20, (d20 lands with the 20 up) is an automatic hit and may even lead to a critical hit.

Attack Bonus
With a melee weapon, the attack bonus is equal to:

Base Attack Bonus + Strength Modifier + Size Modifier + Miscellaneous Modifier

With a ranged weapon, the attack bonus is equal to:

Base Attack Bonus + Agility Modifier + Size Modifier + Miscellaneous Modifier