The built-in MovableType search allows you to search entries and/or comments for user-specified text. However, you have no control over which parts of your entries and comments are searched and when listing search results using the built-in template tags, it is only possible to display the entry, regardless whether its text or that of a comment was matched by the search terms.
The MTSearchPlus plugin enhances the search capabilities in two ways. First, it lets you specify which parts of your entry (i.e. title, text, text_more, keywords) and comment (i.e. text, auther, url) it searches through. Second, it enables your search results to list whether an entry's text or specific comment(s) were matched by the search term. Finally, it only searches visible comments and ignores those pending approval or marked as junk.