The TU Link button will appear by items indexed but not available full-text in the database you are searching. Simply click the green TU Link button next to the book or article you're interested in and you'll be redirected to a different database that does have full-text access.
Not everything is available digitally or it may be trapped behind a pay wall in a database the library does not subscribe to. If that's the case, you can still request the article or book through InterLibrary Loan. Requested books will be made available to you at the first floor Check Out Desk and articles and book chapters are usually provided as downloadable PDFs.