the meaning of "grass dogs (dogs of grass)"
刍狗, is made of grass and used as sacrificial offering, metaphor for humble or useless things.

天地不仁，以万物为刍狗, means heaven and earth, i.e. the nature, is not benevolent and partial, it treats all things on the earth the same and fairly.

It just reflects the ideology of 道家(taoists); 顺其自然(let nature take its course, don't intervene too much).

http://baike.haosou.com/doc/6641911-6855725.html
http://baike.haosou.com/doc/5377628-5613764.html