In TEA, the replacement tables is a something like a set of rules for the text replacing. At the first, you create the file with a table. Here is an example:
cat=dog mew=bow-wow
Then save this file as a replacement table (in UTF-8 encoding!). Just use File > Save different > Save as a replacement table. After that, your table will appear at Functions > Replacement tables menu.
Now we shall try to apply this table. Create a file and write a text like this: The cat says: "mew". Select this text, then select your table from the menu and see what will happen. As a result we have a new sentence: The dog says: "bow-wow". So the replacement tables is a way to do a multiply replacement.
To open your table files for editing, use the file open dialog's TEA special locations drop-down list. If you want to delete or rename your tables, take a look at Preferences > Maintainance page.