It depends on the class of your character.
It’s almost like a “built-in bonus” for things that your character is very good at (i.e. actions they’re proficient in).
Every character has some skills, saving throws, weapons, and abilities that they are good at because of their training and background.
When using one of these skills to roll a d20, each character gets a special bonus called a “proficiency bonus.”
This is a number that is listed for each class in the class table of D&D sourcebooks.
A character’s Proficiency bonus is based on their total level, not the level of their classes.
For example, when a character reaches level 5, their proficiency bonus goes from +2 to +3, no matter if they have more than one class or not.
The proficiency bonus is added to a number of different kinds of checks, but only if the character is proficient in the right skill or ability.
Among these checks are:
(a) Weapon attack rolls
(b) Spell attack rolls (for casters)
(c) Spell save DCs (for casters)
(d) Saving throws
(e) Skill checks (technically this is a type of ability check)
(f) Tool use checks (this is also a type of ability check)
In 5e, it is common for other skills to be based on a character’s proficiency bonus as well.
For example, a Battle Master Fighter’s Maneuver Save DC is partly based on their proficiency bonus, and a Phantom Rogue can use certain abilities equal to their proficiency bonus times per day.
Note that a character’s AC does not go up if they use armor or shields that they are proficient with, but using armor and shields if you are not proficient can hurt you.
Like being able to move slower or not being able to cast spells.
The Player’s Handbook also says:
“Your proficiency bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once when you make the save.”
Take Morgana, who is a Level 1 wizard.
She has a +2 bonus to her skill with daggers, darts, slings, quarterstaffs, and light crossbows.
She has 10 (+0) strength, 16 (+3) dexterity, and 16 (+3) intelligence.
Morgana can use her dexterity to attack with a dagger because it is a finesse weapon.
This gives her a total of +5 to hit (+3 for Dexterity modifier and +2 for proficiency) and d4+3 damage (note, proficiency bonus is not added to damage).
Morgana could also use a light crossbow, which has a +5 chance to hit and does d8+3 damage.
If she attacks with the quarterstaff, she must use her strength (+0), but she can still add her proficiency bonus, giving her a total of +2 to attack.
The damage is d6, or d8 if she uses the property that lets her do a lot of different things.
Morgana could use a rapier, which also has the finesse property, even though she is not proficient with rapiers and can’t add her proficiency bonus to the attack roll.
Because of her dexterity, she has a +3 chance to hit and does d8+3 damage.
In theory, Morgana could use a Great Sword to fight.
But since her strength is only 0 and she doesn’t know how to use a greatsword, her attack roll bonus would be 0 and the damage she does would be 2d6.
Morgana could also attack with a cantrip like fire bolt, which would have a +5 chance to hit (Int modifier plus proficiency bonus) and do d10 damage.
She could also cast “Toll the Dead,” which has a Spell Save DC of 13 for her target (8 + int modifier + proficiency bonus).