It means that the disease or illness in question is hard to detect so doctors do not always spot it straight away (under diagnosed) and it is not always dealt with by doctors when they do manage to diagnose it (under treated). Rare diseases and rare forms of common diseases are often under funded for research and under publicised which leads to the above happening.

What does something mean? What is meant by something? would write "under-diagnosed" with hyphen and "undertreated" as one word.

It means that the disease or illness in question is hard to detect so doctors do not always spot it straight away (under diagnosed) and it is not always dealt with by doctors when they do manage to diagnose it (under treated). Rare diseases and rare forms of common diseases are often under funded for research and under publicised which leads to the above happening.