This function is for administrators and moderators only to stick topics which will be located at the top of the topics list. The topics will not fall down the list if any new posts are added.
Select a topic by ticking the corresponding checkbox, then click on the Action bar when you are in a topics list.
Click on the Action bar if you are already in a topic which you want to stick/unstick.
Click Stick or Unstick from the drop-down menu that appears.
You can see the sticked topic in the top of the topics list.
When creating a new topic, you can also make this topic sticked by ticking the Sticky checkbox in the Options tab.
You can also stick/unstick a topic while editing the topic by ticking/unticking the Stick the topic checkbox in the Options tab.