Punabbhava
The entire poem is about that central Buddhist idea punabbhava, found in the final stanza, which is often translated as "renewed existence" or even "again-becoming." In the process thinking that so characterizes Buddhism, people and things do not "exist" as much as they "become," and this more dynamic form of the verb "to be" is usually preferred in the literature. This life we cling to is merely an episode in a much larger drama of perpetual birth and death, with existence recurring "over and over," and we will never sort it out until we are capable of holding this wider view of our situation.
http://www.accesstoinsight.org/tipitaka/sn/sn07/sn07.012.olen.html