when we open the subroutines we can often read the following information, what's meaning of EQUIVALENCE( UDAT*(*), *)? where * maybe data, or some other nomenclature.

STAR is using it to reveal data in the usrdat.inc file. Variables in that file are sort of anonymous, but the equivalence statements give the relevent variables more meaningful local names. It's just standard fortran though - nothing exotic and nothing to worry about.