The Alerts tab is the place in Purple Mash where you will be informed of things requiring your attention.
You can click on 
in the notification to take you straight to the approval screen for the board concerned and allow you to review the work.
Alternatively you can click on "Tell me more..." to expand the notification to give you additional details of the items requiring your attention, and giving you the option to jump straight to the review screen for a particular project using "Click to open work".
You can dismiss any notification by clicking the 
icon.