The tag tree is a method used by SCREEM to give access to commonly used tags. The program is distributed with an full HTML 4.0 branch in the tree. Also if you have Bluefish (a GTK based HTML editor) installed then SCREEM will build branches of the tag tree out of the PHP3, RXML, and SSI functions that are provided with it.
The tag tree itself is just an xml file, stored in $prefix/lib/screem/screem.tagtree. There is currently no graphical editor for the tag tree at the moment so if you want to change the main one then you will need to load the file into the editor and edit it by hand.
As well as reading the tree from the above location (and the Bluefish function files in $HOME/.bluefish SCREEM will also scan $HOME/.screem for any .tagtree files. If you have any in there then they will be added to the tag tree upon launching screem. Again these files will have to be edited by hand, the format can be found by looking at the main distributed tree file
Clicking on an entry in the tag tree will insert the tag (or function) that is specified. If the text has opening and closing tags then the cursor will be inserted between them ready for you to type. If however you have highlighted some text then the tags will be placed around that text