It means that prior to parsing JSON, parser is given exact path (or paths, or wildcards) it must retrieve, and then it will scan the string in one forward path with minimum possible allocations. It's for cases where you, for some reason, have to process enormous amount of serialised objects as strings, and need to get just a few small things out of them occasionally, and do it in JS.

As it processes input in batches, you can also use it in cases where you don't even need to load the whole input data in memory, if you chose so.