That's the geekspeak for '"sum total" should be rephrased into "summary"'.

Ah I see, thank you. Is it some sort of programming syntax or like, scientific publishing notation?

It's regular expression syntax for, roughly, "substitute 'mary' for ' total'", which transforms "sum total" - " total" + "mary" --> "summary.

They mean "Sum total" -> "Summary"

As in, that's a more accurate descriptor(?)

relevant xkcd: https://www.xkcd.com/208/

Only in the sense that it's something to do with regular expressions.