The central theme of this poem is that the mundane things in life are just as important, if not more so, than the more flashy things that seem to be important. This goes along with a major theme in much of Thomas's work since he was a poet who was very concerned with describing the humble details of country life.
For most of this poem, we expect that the subject is something that is eye-catching and flashy. We are told about cocks crowing and how they cleave the darkness with their silver bows. We are told about "heralds of splendour" and coats of arms. These are all things that seem important and that draw attention to themselves. Up until the last line, we think the poem is about these things.
But then, in the last line, we find that these are not really the focus of the poem. Instead, it is farm workers who are getting to go milk the cows -- a humble task if ever there was one. The poem is saying that these people who seem so unimportant are just as important as any strutting roosters or splendid heralds.
Reading the poem through it could be about the start of a new day. Cockerels crow in the morning and in the past that was when they people they had to start work. The 'sharp axe of light' and 'silver bow' could be a symbol for a ray of sunshine ending the night time. To me the poet is saying the Cockerels are almost calling the new day to them telling the workers to start work.
It may not be right but that's my interpenetration of the poem. Hope I've helped you a bit.