Alerts tell you when some interesting data changes happen in your account. They work by analyzing historical data. Whenever there is a significant change in your data, you will get an email. Also, you get informed in the notification in the notification feed.
Learn more about Alerts in our Best Practice.
On the overview page, you can edit and create alerts.
If you have reached the maximum amount of alerts, you are not able to create a new one. Below the table, you will find the information on that. 
Talk to your account manager if you would like to increase the limit.
The following actions are available for existing alerts:
Trigger: Sends an email showing you the message that would be sent if the alert is set off. This includes a chart that shows the performance in the past.
Edit: Takes you to the configuration mask for an alert to make changes to it.
Delete: Deletes the selected alert.