Almost universally, in almost every text I’ve read, it means “use tools that are well-known to people with an undergraduate degree in a related field, or simple extensions thereof.” (There are some cheeky counter-examples, such as Weil’s Basic Number Theory, but this is far more rare.)

In the same way you cannot expect someone who has never seen a circuit or understand logic gates to construct a processor, you cannot expect someone without any mathematical maturity (e.g. basic linear algebra and calculus) to use QM in any fashion other than understand the most basic of constructions.