This can be a slightly confusing term for some people.
It originates from the organisation Alcoholics Anonymous, which developed a recovery program based around 12 specific steps, which are really statements of experience of what the early members of AA did in order to get sober.
These statements quickly became known as the 12 steps of recovery, and AA became known as a 12-step fellowship.
What happened quite soon after AA started, was that people realised that the principles of this 12-step program could equally be applied to any other problem, addiction or behaviour they have that they could not solve on their own.
A wide range of other 12 step fellowships began to emerge, based on the principles of Alcoholics Anonymous.