It means that it is the appropriate time and place to discuss this issue.

Imaginary example. You live in a U.S. down and you are at the Zoning Board of Appeal meeting to give your input to a developer's proposal to build a two-family house in your neighborhood. You live in an area that's zoned "multi-family residential." The board asks for comments. Someone stands up and says "I'm not interested in seeing their plans. You turn them down. There shouldn't be any two-family houses in our neighborhood and the zoning ought to be changed." The chairman says "Thank you for your input, but this isn't the proper forum. The Zoning Board of Appeal doesn't decide on how areas are zoning, it rules on issues regarding the existing zoning. You need to take your concerns to the Planning Board."

Perhaps you are thinking of "web forum". Consider the etymology of the word "forum". forum (n.) mid-15c., "place of assembly in ancient Rome," from Latin forum "marketplace, open space, public place," apparently akin to foris, foras "out of doors, outside," from PIE root *dhwer- "door, doorway" (see door). Sense of "assembly, place for public discussion" first recorded 1680s. Consider now the modern meaning of "forum" A meeting or medium where ideas and views on a particular issue can be exchanged. The word "forum" does not necessarily refer only to a "web forum".