There are 3 ways to respond to a notification.
Notifications can be found in the notification centre (1) or on the timeline (2) (Comments or events with a icon in the corner of the tile). By clicking on the notification, you will be directed to the Sensor, KPI, Task or Conversations page to get more information why the notification has been sent.
Most notifications are related to a user generated comment or automated event. You can reply to these on the specific page which they relate to, so for example a comment about a sensor, will be found on that sensor details page, and an event about a KPI threshold being crossed, will be found on that specific KPI details page. You can not reply to comments and events shown on the machine page timeline itself.
In the notification centre you can dismiss all notifications at once (5), or individually (6).