alt Hacker NewsThis is standard CS terminology - if you take a compiler course it'll be explained in the first couple lectures.
The ELI5 is that "semantic actions" are bits of code that a parser generator runs, and "production rules" are patterns that tell it when to run them.
I would usually think of a 'parser' as simply something that tells you how to produce the input given a set of production rules.
For a simple ambiguous arithmetical grammar, the output of a parser fed "1 + 2 * 3" might be something like "[Mul [Add 1 2] 3]", or "AddExpr(1, MulExpr(2, 3))", or "1 2 3 Mul Add", or "1. Start with 1; 2. Add 2; 3. Multiply by 3", or (more likely in practice) in-memory data structures corresponding to one of those. The form of the representation doesn't matter; it's just an indication of the sequence of steps that gets you from 'nothing' to whatever input string was provided.
But there's not really a concept of 'actions' to go with parsing. Parsing tells you how some string was constructed, or how it might have been constructed. What you do with that information is up to you.
In the example above, the parser's interest in a MulExpr is that it can produce a string containing the token '*'. The fact that mathematically two times three equals six doesn't come up.