obj is the songpos object. song is renoise.song(), as declared on the third line.
If you want to do a normal song-wide editstep of 2 lines, you would just add the following lines below:
local tmp_pos = renoise.song().transport.edit_pos
tmp_pos.global_line_index = tmp_pos.global_line_index + 2
renoise.song().transport.edit_pos = tmp_pos
Just as an example. You can manipulate the songpos values any way you want. Use it for edit stepping, or use it for pattern iteration and manipulation.
PS. note that the renoise.Song.transport.edit_pos object is a bit obnoxious in that it requires you to feed it a new songpos object to set the edit position.