I am reading the final playtest rules, and I keep seeing how you can add proficiency bonus to Attack rolls checks with weapons you are proficienct and skill checks in skills you are proficient in, but I can't find anything on what this bonus ACTUALLY is. How do you determine what the numerical actual bonus you add to your roll is?
Proficiency bonus is noted in your character advancement table (in Character Creation file) as "Proficiency Bonus", and that is universal for your character level. For example, according to the table, a 3rd-level character gets +2 for proficiency bonus, because the table says so. For convenience of single-classed characters, you can also look up your proficiency bonus for your level in your class advancement table, which is present in the Classes file.
Thus, a 5th-level Fighter with Strength 18(Modifier is +4) wielding his greatsword makes attack roll with
Proficiency Bonus represents your experience from a roleplaying viewpoint, and your power curve from a game design viewpoint.
In DnD3e, for weapon using classes it was your base attack bonus, together with your feat bonus, ability improvement and your magic item it was quite progressive. You gained a level, your attack improved by more than 1.
This math in editions 3 and 4 meant that the threats you ran away from on level 1 became easily trivial. By level 11 your attack bonus, and your defenses were about 10 higher than on level 1. (DnD4 attack increase: 5 from level, 1 from ability, 3 from item, 2 feat)
Basically what was an easy "half your level rounded down" in DnD4e, is now a bit less catchy "5 plus your level, divided by 4, rounded down".