There are 3 ways to respond to a notification.
Click on 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.
Reply to a notification
You can reply to a notification by clicking on the comment or event in the timeline (2) with a icon. Then press the "plus" icon in the lower left corner (3).
Then you can type a response (4). Your reply is visible to all other users. Other users will also receive a notification about your reply if you leave the notification setting on.
In the notification centre you can dismiss all notifications at once (5), or individually (6).