A character's proficiency bonus is a static bonus that applies to any d20 rolls that they are proficient in. Players can be proficient in tradecrafts, abilities, any many other things. However, the proficiency bonus is always the same and scales as a character levels up.
If you train in a class, you are automatically proficient in its abilities so long as you are not wearing armor beyond that class' maximum AV.
As a shorthand, your proficiency bonus may sometimes be written a "P".
The levels at which characters gain increased Proficiency are shown highlighted in the table below.