Text resource are an alternative to hard coded strings in the script files. Generally, you wouldn't use these. They are handy though, if you want an easily translateable game. In that case, you could use text resources for the text, and would only need to change them to make your game into another language.
They are simply a collection of null-terminated strings. Sierra used text resources for such things as displaying text and message boxes.
You can use the text editor to edit text resources.