BLIC ""-//W3C//DTD XHTML 1.0 Transitional//EN"" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
(Note: There seems to be a minor typo in your original line with ".d" at the end, it should likely be ".dtd" for correct translation and application as shown in the corrected version above.)
This line is typically used in documents to declare that the document adheres to the XHTML 1.0 Transitional standard defined by the W3C.