The Help action is kinda squishy in tabletop, since it's got a fair amount of DM discretion built into it.
In simplest terms, you use your action to give another creature advantage on the next 'thing' they do against a particular creature. Usually that's an attack roll, but it's not hard defined.
Solasta has a few context-based help actions where you can just click on the creature needing help, but haven't implemented a full help action with its full flexibility. BG3 hasn't touched it yet, and I don't think they will, at this rate.