The song is about the difficulty and the dangers of Vietnam jungle or the jungle in general which is interpreted through various metaphors and wordplay.
That life's a jungle and that there is the weak and the strong. There are predators and prey and that we are all gods creation. The lyrics of the song strong support these views, for example the lyrics "I'm a tiger when I want love" Conveys the message that he is a Tiger or a predator for love. "I'll write on your tombstone, I thank you for dinner" also support the view of a predator and prey world. The lyrics "And He who made kittens put snakes in the grass" suggests that he is some sort of god or creator that made the creatures on this earth to live in this jungle of life.
Jethro Tull were a British rock group formed December 20, 1967. The group first achieved commercial success in 1969. The song was written and composed to become part of the soundtrack for a movie which was never actually produced. The movie script was a black comedy about a dead teenage girl visiting Heaven and her various misadventures in the afterlife.