A more thorough explanation:

A designated public forum is a public place where people can gather to express their ideas and exchange views. It is different from a traditional public forum because it is not a place that has been traditionally used for public assembly and debate, but rather a place that the government has opened for use by the public as a place for expressive activity. Examples of designated public forums include public-university facilities or publicly owned theaters.

Unlike a traditional public forum, the government does not have to retain the open character of a designated public forum. The subject matter of the expression permitted in a designated 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, as with a traditional public forum.

For example, a public university may designate a certain area on campus as a designated public forum where students can gather to express their views on various issues. The university may impose reasonable restrictions on the time, place, and manner of expression, but it cannot prohibit expression based on the content of the speech unless it meets the strict scrutiny test.