I come from haunts of coot and hern.- The brook starts from a place the coots (a kind of duck) & herns (commonly known as herons) spend most of their times.
I make a sudden sally,-The brook suddenly rushes down.
And sparkle out among the fern,-As the brook flows it sparkles because of sun rays, and it flows through a ground which mostly have grasses and flowerless plants (ferns).
To bicker down a valley.-The brook flows down a valley making noisy sounds.
By thirty hills I hurry down,-Here the brook swiftly flows down many hills. There are not literally 'Thirty hills' but the poet make's the line creative by using 'Thirty' and not 'many'.
Or slip between the ridges,-The brook 'slips'(quickly moves) between long narrow hilltops.
By twenty thorps, a little town,-The brook flows down past many villages (Thorp-Old word for village) -again the poet tries to make the line creative by using 'Twenty'-not literally 'Twenty villages' and a little town as well.
And half a hundred bridges.-The brook flows and passes by/through many bridges-not literally 'Half a hundred' bridges.
Till last by Philip's farm I flow-The brook flows by a farm probably owned by a man named Philip.
To join the brimming river,-After the farm he flows to join a overflowing river.
For men may come and men may go, But I go on forever. Men/people have a short life spam but the brook is immortal so it has a longer life spam and hence goes on 'forever'.
I chatter over stony ways, In little sharps and trebles, -As the brook flows it chatters (makes a interesting and musical sound) over a stony creek bed.
I bubble into eddying bays, -When the brook flows backward it 'pushes' the air and makes bubbles.
I babble on the pebbles, -As the brook moves it makes sound because of the pebbles.
With many a curve my banks I fret-The brook flows curvily because at one point the path curves and it wears away.
By many a field and fallow,-The brook flows...