I'm so fucking hyped. Can't wait for the actual gameplay reveal later.
As for the people who haven't played D&D 5E, I'll summarize some basic things about 5E (the pen-and-paper, not this game's probable adaptation of it) combat:
- It's obviously turn-based
- There's a number of things you can do in each of your turns: action, bonus action and movement are the basic ones, the crux of the "turn (or action) economy" of the game.
- You can use (unless explicitly stated otherwise) your action once per turn. Same with your bonus action - only once per turn.
- One of the most common actions you can take is the Attack action. Some classes allow your character to start attacking more than once per turn once you reach certain levels - but, make no mistake, this "Extra Attack" is still one Attack action. This means that if your character has the "Extra Attack" feature it has to use one action (Attack) to hit the enemy twice - your character cannot use a physical attack followed by casting a spell under the "Extra Attack" feature, because casting a spell is a different action than Attacking.
- Spells have casting times: some take one action, others take a bonus action and others take a reaction. That means that, if you have spells slots available, you can cast two spells on the same turn: one that has a casting time of one action and one with a casting time of one bonus action. There's a pretty big limitation, however: if you use a bonus action spell, the action spell has to be a cantrip (which are "minor" spells, so to speak. I won't go into detail about it here.)
- As mentioned before, there's also reactions, which are "special actions" that can occur on your turn or someone else's, and essentially they are instant responses to a certain trigger. For example, the Shield spell has the casting time of one reaction, and you must cast it when your character is hit by an attack and lasts until the start of your next turn.
- Movement is always available to your character unless explicitly stated otherwise (for example: your character is paralyzed, or restrained). Your character can move up until your speed (as it appears on your character sheet, and might be affected by spells or certain equipment) in a single turn, and you can break up that movement however you see fit within your turn. If your character has a speed of 30ft, for example, you can move 10ft, use the Attack action and then move the remaining 20ft away, if you want to.
- Apart from the Attack and Cast a Spell actions, some other actions you can take in your turn are: Dash, Disengage, Help, Hide and Ready.
- The Dash action means you gain extra movement in your turn, equalling your speed (so if your character has 30ft speed and you choose the Dash action, it now has 60ft for that turn), but -considering it's an an Action- you cannot attack during that turn (unless the attack uses a bonus action as explicitly stated)
- The Disengage action means your movement doesn't provoke opportunity attacks during that turn. Opportunity attacks are triggered when a hostile creature moves out of another creature's reach - so, for example, you are melee attacking a goblin and then you want to flee 10ft away from it, then that movement triggers an opportunity attack against you (which is like any normal attack in terms of dice for that creature) before you actually can move. Also, opportunity attacks use a reaction. So, anyway, using Disengage can stop that from happening.
- The Ready action gives you the chance to "hold" the action you want to use until a certain trigger happens (for example, "I'll hold my melee attack until this enemy gets within 30 feet of me"), and thus it'll use your reaction if it happens before the start of your next turn. If the trigger doesn't come to pass, then it's wasted :P
That's a very basic primer on turn/action economy on 5E. Hope it's clear enough, and we'll see how that is implemented in BG3!