In typography, a closed character or symbol is one which has been filled in, or is solid, such as a closed bullet. An open character or symbol is one which exists essentially as an outline, such as an open bullet.
The open/closed dichotomy is also used to refer to the typesetting of dashes. In this case, closed refers to a dash set without a word space at either end (such as "word—word") while open refers to a dash set with a word space at either end (such as "word — word"). On older typesetting systems, open dashes were preferred, as they allowed the typesetter more places to break lines (typesetting systems tend to look for word spaces as places to break lines), but more sophisticated devices know enough to break lines at either end of an em or en dash.