Question

perennial natures of the brook.

Answer 1

It says that men takes birth and die but it is going on forever i.,e
       
                    For men may come and men may go, 
                             But I go on for ever. 

It is the perennial nature of the Brook

Answer 2

For men may come and men may go, 
But I go on for ever. 

In his poems, Alfred Lord Tennyson skillfully composes the music of the English language into harmonious verbal melody. His poem "The Brook" extols the talents of the stream that moves from a mountain and meanders through grassy areas, fields of flower, brambles, to the "shingly bars" where it finally joins the river.  While men live and die, the brook survives and "goes on forever," attesting to the infinite power of nature. So, in "The Brook," the refrain is thematic, as well as melodious.