I wanted to give a definition of my own. But, seems like Investopedia has a better one:
Investigative activities that a business chooses to conduct with the
intention of making a discovery that can either lead to the
development of new products or procedures, or to improvement of
existing products or procedures. Research and development is one of
the means by which business can experience future growth by developing
new products or processes to improve and expand their operations.
So, people working on researching and experiments of products, which are yet to go public are what crudely defines R&D engineers.
For example: A data engineer builds the data stack and architecture of the company, so it would not be called an R&D position/job.
However, a data scientist works on AB testing, research about the marketing growth, etc; so they are R&D engineers, and so are engineering working on products which are yet to be made public and still under development, receiving active criticism from the beta testers.