What exactly does the Attack Bonus on weapons apply to?
Completely new to D&D; myself and some similarly inexperienced players jumped in with D&D 5e, using the premade characters for the starter set that was released. We searched around but couldn't find an answer to something:
What exactly is the Attack Bonus listed on the weapons? Is it a modifier you add to your attack roll or your damage roll?
The Attack Bonus you're referring to, in the middle of the Starter Set Character Sheets, is added to attack rolls (only, never damage). It's calculated using your Proficiency Bonus plus your relevant Ability Modifier (usually Strength for melee weapons and Dexterity for ranged). The Damage is included in it's entirety just to the right of the Attack Bonus, in the same "Attacks and Spellcasting" box.
For example, the Human Fighter character comes with a greataxe, with an attack bonus of +5 (and so rolls 1d20+5 to hit) and a damage of 1d12+3 (and so rolls exactly that for damage on a successful hit).
@smashtastic this question specifically is about the free "Basic Rules" document and the sample characters in it, but the top answer here cites page references from the PHB for calculating your total attack bonus with a weapon. Ignore the fact that it's marked as a duplicate of this question; these questions are similar but not the same.
I assume that you're talking about the +1 that you're seeing on certain weapons in the starter set.
These are magic weapons. This +1 is indeed applied to the attack roll and the damage roll. A magic weapon may also have other properties (like the axe in Thundertree that has advantage against plants).
Unless, you're referring to the +4/5/7 on the character sheets. This only applies to the attack roll. A weapon attack is represented with two different expressions. Here's the Greatsword from the Ranged Fighter:
Greatsword +4 2d6+2 slashing
I'll break that apart now. The first entry is the weapon type. You can by different weapons, this has two effects, if you're proficient, your attack modifier only changes if it's a finesse weapon and you decide to use Dex for the attack instead of Str. But your attack roll is your proficiency modifier (+2 at L1) plus your ability score (the ranged fighter's starting Strenght is 14 so the modifier is +2).
The second entry is the attack bonus, we just calculated that above. It's determined by whether or not you are proficient (the fighter is proficient with all weapons, other classes less so, see the box with "proficiencies" listed for the weapons you can add this bonus to). Then you add the appropriate ability modifier. Melee attacks use Strength by default, but finesse weapons can use Dex (not required). Ranged weapons use Dexterity by defualt, but thrown weapons can use Strength (not required).
Finally you have the damage. This is the damage dice indicated by the weapon with the ability modifier you're using for the attack added to it (so the Greatsword has 2d6 from the sword and +2 from your Strength modifier).