Actions Explained

Actions in Combat

When you take your action on your turn, you can take one of the actions presented here, an action you gained from your Race or a special Feats/Powers, or an action that you improvise. Many monsters have action options of their own in their Sheets
The DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure

Attack

The most common action to take in combat is the Attack action, whether you are swinging a sword, firing an arrow from a bow, or brawling with your fists. With this action, you make one melee or ranged attack

Cast a Spell

Spellcasters as well as many monsters, have access to Spells and can use them to great effect in combat. Each spell has a casting time, which specifies whether the Caster must use an Action, a Reaction, Minutes, or even Hours to cast the Spell. Casting a Spell is, therefore, not necessarily an Action. Most Spells do have a casting time of 1 Action, So a Spellcaster often uses his or her Action in combat to cast such a Spell.

Dash

"When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash. Any increase or decrease to your speed changes this additional movement by the same amount. If your speed of 30 feet is reduced to 15 feet, for instance, you can move up to 30 feet this turn if you dash."

Disengage

If you take the Disengage action, your movement doesn’t provoke opportunity attacks for the rest of the turn

Opportunity Attacks

If you mess up or something like that it will lead to a "Opportunity Attack" [or vice versa]
Creatures roll advantage on Opportunity Attacks

Sneak Attacks

When you attack a Creature that does not know you are there, It counts as a "Sneak Attack"
It deals (1D4) amount of Damage
1 = 1.5 Points Of Damage
2 = 2 Points Of Damage
3 = 2.5 Points Of Damage
4 = 3 Points Of Damage