As in literally not just the north star, but six other stars
Literally: Tai – mountain – north – dipper
Alternately: A leading and respected figure in a given field; someone of immense personal authority by whose standard others chart their courses and measure their progress. The human equivalent of a famous mountain or constellation.
Notes: Cartographical in feel, this is a compound of proper nouns: 泰山 is simply the name of a famous mountain, Mount Tai (a.k.a. Tai Shan), while 北斗 is the Big Dipper constellation.
This phrase comes to us from the New Book of Tang, or Xīn Tángshū, (Japanese 『新唐書』 = Shintoujo), essentially a heavily-revised replacement for the history text that we retrospectively call the Old Book of Tang, which we’ve met before. It seems to be describing Han Yu (韓愈, Japanese Kan Yu), whom we have definitely met before.