
*You only add your proficiency bonus if your hero has been trained for that task*
For instance: Say the DM explained to you that a brigand is firing an arrow at your character from atop a hill. Your character, let's say a typical knight in armor, charges up the hill to defeat them. We can assume running up the hill goes according to plan and not ask for a roll, as that's a fairly mundane task.
Once you get there, you swing your sword at them. Here the DM will ask for a roll of your 20 sided dice because hitting your enemy is tricky business. Then you'll add your strength modifier because it takes strength to swing a sword and also your proficiency modifier because your knight is clearly trained in using a sword.
Now if this was a bar fight and you didn't have that sword, you might find a nearby chair to hit your enemy with WWE style. Since you certainly aren't trained in chair to chair combat, we wouldn't add the proficiency bonus, so it would just be the 20 sided dice roll plus your strength modifier.
Examples of each type of action are below:
Move: Stand up from prone, run your movement speed, swim or climb at half your movement speed. (Your speed is located in the column just to the right of your skills.)
Standard: Make an attack, cast a spell, use an object (throw a grappling hook or spread marbles on the floor to make someone running behind you slip and fall)
Bonus: Some experienced heroes have learned to do something so fast they can squeeze it into a round on top of everything else. In the case of a martial artist, she can use her bonus action to land an extra punch or two. In the case of a rogue, he can use a bonus action to move a second time or hide
As your hero becomes more experienced they will learn how to cast spells and perform feats. These special abilities have their own unique text which is found in the player's handbook.