At the bottom of some threads, you may notice some keywords. These are the thread's "tags".
If you click on a tag at the bottom of the thread, it will give you a list of threads which have been given the same tag.
The aim of tags is to make connections between threads which share a common theme and touch on a common subject. For example, if you click the keyword "tags" at the bottom of this thread, you will be given a list of other threads which also have the tag of the keyword "tags".
Tags on orthodoxchristianity.net are generated by the posters themselves. You can add a tag to an existing thread by clicking "Add Tag" at the bottom of the thread. And whenever you start a new thread, you have the option to add tags in a field below the field where you enter the subject title.
A good tip is to first look at the existing tags on the forum before tagging a thread. Tags only work if the same word is used to tag corresponding threads. For example, if you start a thread about vigil lamps and use the keyword "oil lamps" as the tag, your thread will will not have the same tag as other threads with the tag "vigil lamps", and so, will not appear in the list when someone click the tag "vigil lamps".
Feel free to tag any thread in order to make logical connections with other threads, but please don't abuse the system. Abusing tags in ways contrary to forum rules is not welcome and will lead to the same consequences as if it were a post which contravened forum rules.