On your bottom right navigation bar, one of the menu items you will see is Alerts:
These are to give you a visual cue of upcoming activities you have scheduled. Alerts are displayed only for timed events and the alert shows up 30 minutes prior to the time of the event. When the Alerts menu changes to red and displays a number in parenthesis (as in our example above), this means you have a scheduled activity in the next 30 minutes. Clicking 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:
Alerts will disappear after their Start time has passed.