What is the operator "" in C++?

I fell on this page where the author talks about the standardisation of the operator "":

The decision of the C++ standards committee to standardise operator "" was [...]

What is he/she talking about? I can't find any information about this, and I don't understand what it could imply (overload for constant strings? Or something more conceptual, that doesn't affect the final use of the language?)

Those are user-defined literals. They allow you to create stuff like std::string, std::chrono::durations or any user defined type (you can make your own literals) in place:

auto str = "Hello"s; // str is std::string("Hello")
auto sec = 5s;       // sec is 5 std::chrono::seconds

A list of the literal-operators provided by the standard library and their documentation can be found at the bottom of the documentation page I linked.

It's the user-defined literal operator which will allow the introduction of new literal syntax based on existing literals.

For more information, show this reference link.