Shodan, literally, beginning step, is the introductory "black belt" rank in many Japanese budo. (It is also sometimes referred to as "first 'dan'.") It is part of the kyu/dan system common to gendai budo.
For most modern-day students, it requires between three and six years to reach the rank of shodan. This varies according to situation and other factors, like the talent of the student, the rigor of the instructor and curriculum, and (perhaps most of all) the amount of time and energy the student spends on his or her training.
As the transition from white (or colored) belt to black belt, the shodan rank often holds special symbolism for martial arts students. The particular significance attached to it has varied and does vary based on circumstance. One often-quoted summary is that a first degree black belt has a thorough grounding in basic technique.
Popular media often fails to distinguish between the various levels of black belt, and thus some have the impression that passing a shodan examination somehow qualifies the recipient as a master of the martial arts. Most consider this mistaken. While an important rite of passage, it is probably more accurate to say that a shodan is ready to begin serious training.
As a black belt wearer, or 'yudansha', a first-degree black belt is often asked to take on some teaching responsibilities. This is normally in an assistant capacity.