The proficiency bonus represents your experience from a role playing viewpoint, and your power curve from a game design viewpoint.
The Proficiency Bonus reflects via game mechanics how much better a character is at some things like attacks due to the level of experience.
Formula to calculate proficiency bonus.
Every class has the proficiency bonuses listed in the class table. It’s 2 at level 1, 3 at level 5, 4 at level 9, 5 at level 13 and 6 at level 17.
You can also use the formula below.
Make sure to round up your answer.
Example:
Calculate the proficiency bonus at level 17.
Therefore, the proficiency at level 17 is 6.