Daggers are melee weapons that have the finesse property. The finesse property allows an attacker to use their dexterity modifier for their attack bonus and damage bonus, in place of the strength modifier.
In this case, the tiefling has a +3 proficiency bonus and a +2 dex modifier, so the attack bonus should be +5 with a finesse weapon, unless there is something missing from the sheet that explains the extra +1 to bring it to +6 such as the dagger being a magic weapon or something. That doesn't appear to be the case here because the equipment for the character is listed on the sheet but something is not adding up correctly, or else I'm missing something on the sheet completely.
As for the piercing damage question, piercing is the type of damage the dagger does. The table in the equipment chapter that details the weapons tells the reader what damage die a weapon uses and also the damage type for the weapon. A dagger does 1d4 damage, and the damage type is piercing. The damage bonus is calculated by adding the attack modifier (in this case dexterity because the weapon has the finesse property) plus any other modifiers to the damage (in this case, none), giving a total of 1d4 + 2.
I'm not certain why the sheet is showing the attack bonus as +6 to hit. All of these sheets seem to have an extra 1 that I can't figure out the source of. If someone finds it, please comment because it's really bothering me and I don't know if I'm just missing something plainly obvious.
I'm leaning towards the calculations on all of these sheets being wrong and chalking it up to an error on the first sheet not being checked and being carried over to every other sheet after. I can see no reason why the bonus is +1 higher than it should be, especially since the spell attack modifier is calculated correctly using the warlock's charisma bonus + proficiency modifier.
Best Answer
This is true for most melee weapons, but not the dagger.
Your damage will be partially determined by the weapon you use. If you're using a quarterstaff your damage would be 1d6 - 3 (one-handed) or 1d8-3 (two-handed) thanks to its versatile property.
Finesse weapons, such as a dagger, may use dexterity instead.
Some melee weapons (such as the dagger) have the finesse property. For these weapons, you may use your dexterity modifier instead of your strength modifier.
Ranged weapons use your dexterity modifier.
If you're using a ranged weapon, such as a sling or crossbow, you'll use your dexterity modifier instead of your strength modifier.
Spells typically do not get a damage modifier.
If you're using a spell, the damage is based on what the spell tells you to use. If it doesn't ask for a modifier, you don't get one. Most spells that do ask for a modifier use your "spell casting modifier," which is intelligence for wizards.