An Alert is sent to you when an Event satisfying a certain set of rules occurs and you have been assigned as a Recipient of that event. This My Alerts page lists the alerts you have been sent.
You are also likely notified some other way, such as by email. This page lets you acknowledge that you received the alert. You can do this in two ways. If you want to delete one or more individual alerts, check the "Acknowledge" checkbox for the alerts, then click "Acknowledge Checked". If you want to acknowledge all of your alerts, click the "Acknowledge All" button.
Click the "Return" button if you want to return to the last page you were on.