mark reviewed
 

what does the 'mark reviewed' icon / instruction do?

it doesn't seem to do anything

Here's a quote of mine from another thread.

I've gone through my projects and set the appropriate time between reviews (x weeks or x months). Then, trying to do this consistently once weekly, I use the filters ( active projects; Next review; unsorted; remaining; any; all) and look at each project for the next week (at times I then look at the next month). Once "reviewed" I hit the "Mark Reviewed" tool and the project disappears; I go on to the next.

"Mark Reviewed" basically sets the review date on the currently selected project to 1 [review interval] in the future. So if your project is set to be reviewed weekly (the default), and you mark it reviewed today, it will next be due for review on Tuesday, July 18th. You can view this information in the Project inspector.