It’s a pretty vague term, but most often is used to refer to one or more of the following roles:
Quantitative Analyst
Quantitative Researcher
Quantitative Developer
Typically, math and CS-heavy, and work on inventing and/or implementing novel algos and models in the finance space. They make a lot of money, and the roles typically require grad degrees (lots of them are PhDs in CS/DS, Stats, Math, Physics, or an Engineering discipline).