It can mean a variety of things. Basically, any time the firm has hired a lawyer that does not fit squarely into the traditional partner or associate role. A few examples:
1. A non-equity partner. Basically an associate who they didn't like enough to make partner, but thought was useful enough to keep around. Too senior for the "associate" label. Some firms don't have non equity partners so they can't just call them partners.
2. A lawyer the firm consults with on an occasional basis. For example, many professors are "of counsel" at firms.
3. A semi-retired lawyer (often who retired from another firm) who wants to work on an occasional or part time basis.
4. A lawyer hired for a special purpose other than billing hours. Perhaps someone with certain political connections, etc. But not working enough for the firm to justify "partner" status.