5e Weapons - "To Hit" attribute, what rule is this referring to?

On the 5e character sheet, when you are editing a weapon, there is a field titled "To Hit".  I can't work out which rule this is referring to.  For example, a mace has a value of 4, while a heavy crossbow has a value of 2.  Do weapons have their own hit bonuses?  I can't find this in the PHB

Proficiency based on level, 1st level is +2. Mace uses strength modifier, i am guessing a 14 or 15 strength, +2 Crossbow uses dexterity modifier, i am guessing a 10 or 11 dexterity, +0

This is different for PCs and NPCs. PCs follow the Ability+Proficiency+Magic Bonus rule. NPCs get a flat number, since many NPC statblocks do not necessarily follow character build rules. In fact, the original statblocks in the source books just give a flat number (regardless of how it was created), so the NPC sheet in Roll20 just transcribes that number rather than trying to calculate, to avoid inconsistencies.