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.
Calculate the proficiency bonus at level 17.
Therefore, the proficiency at level 17 is 6.