When a player types in a word that has a synonym defined, that word is converted to its synonyms - this makes setting up Custom Player Commands much easier, as you can cater for commands like "read book" and "read novel" by just setting up one command for "read book". If you then set up a synonym so that the word "novel" is converted to the word "book", then when the player types "read novel", it will be automatically converted internally into "read book", and so the command for "read book" will be run.
(Note that you can also achieve the same effect by adding the word "novel" to the Other Names window of your "book" object)
To add synonyms, select "Synonyms" from the tree menu. Here you can add and edit synonyms by specifying one or more words to convert from, and one word to convert all of these words to.
In the example above, you'd add "novel" to the "Convert..." list in the Synonyms editor, and "book" to the "to..." box.