The th refers to aspiration, as in the difference between the /tʰ/ in "top" versus the /t/ in "stop". Put your hand in front of your mouth and say "top stop top stop", you can feel the unwritten "h" in "top".
It's not referring to "th" (dental fricative) sound such as "thin"
It's not referring to "th" (dental fricative) sound such as "thin"