"to worship the ground sb walks on" = to show sb an enormous amount of respect and admiration. tread, trod, trodden = an old-fashioned verb meaning "walk". It's seldom used in modern English, except in idioms like "tread lightly". I have to say, the word "fairly" looks strange to me there, but perhaps it has to do with the context...

Basically, it means they showed a high amount of respect. (:

It could mean that the reason they live is to work the land that they walk on. If it were not for that land in which they worked, the slave has no value, and is therefore killed as a result. The land is what keeps them alive; they see it as their savior, so they worship it as such. It would be better to google this or look it up in a book.