The title of Tennyson's poem "The Brook" gives the greatest clue as the what the poem is about. The poem personifies a brook (personification- giving of human characteristics to a non-human/non-living thing) by giving the brook the ability to describe itself. Outside of the entire poem being one of personification, the following line is a specific line exampling the poetic device:
I chatter over stony ways.
A brook cannot chatter (to talk incessantly/continuously), but people can. This makes the line contain personification.
As for the theme of the poem (know that themes of poetry may change given subjective thought of a reader), the theme of this poem is persistence. The brook, regardless of what it passes, will continue on "forever". No matter what bumps it must pass over, no matter the twists and turns, the brook will sustain.
One could look at the poem as one of mirroring life. No matter what the challenges a person will face, with persistence one can conquer all.