On your Top Right Menu Bar, one of the menu items you will see is Alerts, represented by a bell icon:
These are to give you a visual cue of upcoming activities you have scheduled. Note: you can also set your Notification Preferences so that you hear an Alert Sound when new Alerts pop up and select from several options as to how early before the event this happens. Alerts are displayed only for timed events. When the Alerts icon changes to red and displays a number in parenthesis (as in our example above), this means you have a scheduled activity in the next 15, 30, or 60 minutes (depending on your Preference setting for Alerts). Hovering over Alerts will display your upcoming Activity or Activities.
If you then click on an Activity displayed in your Alerts, it will open up the Activity for editing or completion.
Alerts will remain until you Dismiss them using the button within the Notification itself or until you've completed the Activity. That said, once you've viewed an alert, it will no longer change the appearance of your Alerts icon or be represented by a number there.