This 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 assume that they refer to what you do when you write a Yacc or Bison file or any other input file for a similar parser generator.

A Yacc file contains the grammar of the language to be parsed, but to each grammar rule you can attach a C function that will be invoked by the generated parser when matching that rule.

That C function is the "semantic action" registered for the corresponding grammar production rule.

For instance, by registering appropriate "semantic actions" the generated parser can become a language interpreter, e.g. when matching a multiplication expression a function that does multiplication is invoked, and so on.

If the parser is intended for a compiler, not for an interpreter, the "semantic actions" may generate some intermediate format of the compiled program, like that used by LLVM.

Others here already answered your question. In the blogpost, the text that immediately follows already demonstrates the concept, then it contrasts to what's done in Pratt parsing (semantic actions to to terminals instead of non-terminals). Non-terminals is another way to refer to production rules. This difference is a pretty big deal so it's important to mention. The rest of the section goes on to explain the consequences of this difference. None of this is mentioned in other Pratt tutorials that I read before so I doubt a slop machine would write articles of Pratt parsing even mentioning this trait at all.

The article doesnt read like slop. And this is a very clear sentence to anybody who knows any amount of parser theory.

"Production rules" are rules in grammar descriptions for tools like yacc. "Semantic actions" correspond to pieces of code/logic executed whenever a given rule is fully parsed from input.

That is not slop btw

The blog reads like organic human text, not mechanical slop.

The terminology is baroque, but pretty standard in parsing circles.

Essentially, different parsing algorithms "run side effects" at different stages of the parsing algorithm application, which can make certain things easy or difficult depending on the timing.

E.g.: If parsing runs side effects before it has 100% decided on what rules have been applied, it may need to "back track" and undo side-effects. This can be very painful to code around, so a common solution is to build (only) a temporary abstract syntax tree (AST) in a pure functional manner, and later come back and process that instead of trying to handle every "do and undo" scenario during initial parsing.

Parsers that only run side-effects when a decision is final allow very efficient compilers that directly "do the thing" instead of building temporary structures.

IIRC, Pascal was compiled this way, in a single pass.