What the Holland Code is: six kinds of work
The Holland Code comes from a simple idea first described by psychologist John Holland: people and work environments can both be described by the same six types, and people tend to be happier when the two match. A person who loves working with their hands will feel at home in a workshop and restless in an office full of paperwork. The code is a way of naming that fit, so you can look for work that suits how you like to spend your days.
The six types are Realistic, Investigative, Artistic, Social, Enterprising, and Conventional, and their initials spell RIASEC. Everyone has some of each type in them. The point is not which types you have, but which ones come first. Your top types describe the kinds of work you are drawn to, and the order between them describes how you like that work to feel.
The code has been used in career counseling for decades, in schools, universities, and career centers, because it turns a vague question — what should I do with my life — into a practical one: which of these six kinds of work do I enjoy most? It does not measure ability. It measures interest.