I am unclear as to what this means. I get a Mythos card that says there is activity at some streets. I put a marker there, but nothing ever seem to happen in the game at that location. Am I missing something here? How does this get played?

Can you name the actual card please or give a more accurate example?

Activity markers are used as reminder for places where something special is happening. Usually a street area you have to be in to pass a Rumor, or an area (street area or location) where some special effects due to the active Environment card can be triggered.

So, Mykull, the marker actually is just this. When you end your movement in a space with a marker, you can check on the linked-to-that-marker Mythos card what's going on.

Let me know if this clarifies your doubts

In the mythos card that says there is activity at some streets, there is always an explanation for what the activity is in the text of the mythos card. For example, there's a mythos card that says, "the first player to end their movement at South Church is blessed." This mythos card would show "Activity at South Church."

Also, most Rumor mythos cards involve an activity marker where players have to spend their encounter phase at a specific location to spend or sacrifice certain items to pass the Rumor.

I should have looked closer. This particular card had an environment which referenced something going on at that location which the players could have done.

Thanks for the help.

This was the one thing that got me stuck, but now reading this, it makes sense.

We tend to put the card on the street in question as a reminder that you can go to the yard sale (or whatever) at that location.