Cut means "if you matched this far, don't consider other options even if the current option fails to parse".

With this grammar:

    expr: expr '+' term | term

And this input:

    5 +

It will first consider the first alternative (expr '+' term) but will fail to match that branch because it's missing the "term" after the "+". That's fine because there's another alternative to try (term). That alternative doesn't match either, and you'll get the error message from the last alternative that it tried (probably something like "unexpected +").

But with this grammar:

    expr: expr '+' <cut> term | term

Then with the same input ("5 +") it will consider the first alternative, fail to match, and then fail immediately with the error message from that first alternative (something like "missing term after +").

yeah, I get the purposed point of a "cut" but how I put one? Exist a basic sample? (ie: Using a peg parser how define what is a "cut"???)

    expr: expr '+' API_call_here? term | term

Hmm, if you're using a peg parser that doesn't directly support this you can emulate it by choosing between the rhs of the cut and a production that will always match

so

  expr : atom (op <cut> expr)*

Would be replaced with

  expr : atom (op (expr / cut))*
  cut  : "XXX"?

or perhaps simpler

  expr : atom (op expr?)*

Either way when you process the expr rule there will be an empty match where the rhs should be, and an appropriate error can be generated

Drothlis comment describes it well, but to perhaps explain where the term comes from, Prolog has a related concept and calls it the cut operator