When someone without experience starts out on a job, on low fire, sometimes with guidance. A phase before becoming a fully qualified worker.
To be an expert (מומחה) you need to have an intership. מומחה and התמחות comes from the same root.
It can also be the first year of medical training after medical school. In the states it is called an Internship התמחות. And the second phase is called residency, which is STAJ in Hebrew.