A suggestion
“The difficulty is that all art is both abstract and suggestive at the same time. You can't show everything. If you do, it's no longer art. Art lies in suggestion.” Robert Bresson
In as far as extracting any meaning from this quote by Bresson- said in an interview in discussion of his film, Au Hasard de Balthazar- it’s referring to the context in which we speak about art as an allusion or a puzzle left for the viewer to complete when questions are left open.