The short answer is that a locality may impose "reasonable time, place, and manner restrictions." Such a restriction, to be reasonable, must be content neutral, serve a significant government interest, be narrowly tailored to further that interest, and leave open ample alternative fora.
So, for example, if you want to lead a large march down a busy street, you generally can't do that in advance without a permit. The reason for this, I take it, is fairly obvious. If a government wants to restrict your ability to conduct a large protest in, say, a public park, on the other hand, they're going to have a harder time in court explaining why the restriction is reasonable.
Courts review these restrictions under "intermediate scrutiny" which means that, while they don't look as hard as they would if the restriction were content based (for example), they are still supposed to conduct a meaningful and detailed review.
So, for example, if you want to lead a large march down a busy street, you generally can't do that in advance without a permit. The reason for this, I take it, is fairly obvious. If a government wants to restrict your ability to conduct a large protest in, say, a public park, on the other hand, they're going to have a harder time in court explaining why the restriction is reasonable.
Courts review these restrictions under "intermediate scrutiny" which means that, while they don't look as hard as they would if the restriction were content based (for example), they are still supposed to conduct a meaningful and detailed review.