Action economy

This is a term used to discuss the options a player has available in a single turn.

Dungeons & Dragons allows abilities to be used in any order that the player wishes:

• (Regular) Action. 

  These are the major things a character gets to do:

  • Casting a spell
  • Activating a magic item
  • Attacking

  Most actions will lead to someone rolling a $D20$ or some damage dice.

• Bonus action. 

  • Set a character up for strategy
  • Bolster a given course of action.
  • Use a secondary weapon

  Nobody gets blanket access to bonus actions; they are dictated by circumstance, class, race, and/or magic effects in play.
  No matter how many bonus actions a character may have access to through careful tweaking of their class, race, and feats, that character may only use one of them during their turn.

While every character can take each, actions are basic and broad enough for anyone to take, but bonus actions are limited in scope and not something granted by default.

• Movement action

  This is the distance a character can move before or after taking their action or any bonus action.
  It can be distributed and divided at will around any other parts of the turn.

• Item Interaction

  Once per turn, not considered a proper action.
  Every character can interact with one object during their turn.
  This means that a character can be in the middle of combat and still:

  • Drawing a weapon
  • pick up a weapon
  • open or close a door
  • light a torch
  • drink a potion
  • sheathe a drawn weapon
  • put away a weapon
  • and so on.

Reactions & Actions Out Of Turn

There are a handful of ways for an action to occur outside of the player's turn via initiative.

One sort of out-of-turn action would be reactions. These are reflexive options that a character can take in response to some triggering incident like being attacked or near someone trying to cast a spell. 

While some classes, feats, and races give access to certain reactions, everyone has access to an “attack of opportunity".

For example, should a character willingly move from inside of your character's melee reach and beyond that distance, your character could use a reaction to make an attack of opportunity and attack with a melee weapon.

There are other actions that, while they are announced during the player's turn, have effects that linger until the beginning of their next turn. The most prominent example of this sort of action would be Dodge, which raises the PC's Armor Class, or Disengage, which is actively trying to leave combat.

Lastly, there are readied actions. A readied action is an action that a player declares happens under certain circumstances. Should the declared trigger fail to occur before the player character's turn comes up on the following round, then the action does not occur. 

Does Wielding Two Weapons Grant a Bonus Action?

$Yes.$

You do get a specific bonus action that allows you to make an attack roll with the secondary weapon, considered an “off-hand attack.” Off-hand attacks do not gain the benefit of the wielder's Strength or Dexterity modifier, whichever may be relevant, to the attack's damage roll upon a successful hit.