A more thorough explanation:

A nontraditional public forum is a public place where people gather to express ideas and exchange views, but it is not a place that has traditionally been used for this purpose. Examples of nontraditional public forums include public university facilities or publicly owned theaters.

The government can regulate a nontraditional public forum, but any regulation must be narrowly tailored to serve a significant government interest and must usually be limited to time, place, or manner restrictions. For example, a city may regulate the use of a public university facility for a political rally to ensure public safety and prevent disruption of classes.

Unlike a traditional public forum, the government does not have to retain the open character of a nontraditional public forum. The subject matter of the expression permitted in a nontraditional public forum may be limited to accord with the character of the forum, and reasonable, content-neutral time, place, and manner restrictions are generally permissible. However, any prohibition based on the content of the expression must be narrowly drawn to effectuate a compelling state interest.

For example, a public university may open its facilities for use by student groups, but it may limit the subject matter of the expression to topics related to the university's educational mission. The university may also impose reasonable restrictions on the time, place, and manner of expression to ensure that classes are not disrupted.