Apprenticeship is a training process where the student of a subject gets to put his theoretical knowledge into real practice by getting opportunity professionally in an industry. The apprentice is exposed to the real job and work condition, where they learn on the field taking actual tasks similar to the permanent jobholders working in the same office.
Apprenticeship is usually a yearlong programme with a firm or establishment where after completing it they are eligible to practice it full time as an occupation. In the eyes of government such labour is known as skilled labour and have precise knowledge of their domain.
A mechanic, Doctor, CA, CS etc all go through this process before they are eligible to do their own practice.
One must not confuse apprenticeship with internship though. An internship is a certificate and is for brief period that may not guarantee a job. It is optional and is not mandatory. But, apprenticeship is a training process and is mandatory step before the person becomes an actual practioner of their skill set.