All the activity streams have a green check box next to the stream title.

 

 

I thought at first that this was to mark as read, but I tested and that's not it. What does this box do?

Easier to determine if you're not on a touch device

 

Pencil: Edit the stream title (when using your own custom streams)

Garbage Can: Delete the stream (when using your own custom streams)

Checkbox: Make the stream your default stream

6 minutes ago, Squid said:

Checkbox: Make the stream your default stream

So when I login to the forum I'll be directed straight to this stream?

It changes the button in the top right to match whatever your default stream is

Thanks.