dh means Denavit-Hartenberg notation
It calculates placement of axis of rotation/direction of translation of a robot in system of coordinates creating new system of coordinates for the next joint to be attached. Its variables have many names. Following this wiki article they are respectivety - 'x', 't', 'a', 'al':
https://en.wikipedia.org/w/index.php?title=Denavit–Hartenberg_parameters&oldid=1070964199