The checkbox to each topic

May I know what for the checkbox to each topic which I can tick and untick?

It doesn’t appear like that to me o.O

Are you sure?

Strange, I can’t see the checkboxes now.

Yes, I’m sure. Weird indeed.

I found a bug in Discourse.

You have to click Messages and toggle on the checkbox. Then click Activity and Topics, you’ll see the checkbox next to each topic.