Ted Hughes was a famous modern poet from England. He lived from 1930 to 1998, and he also wrote plays and stories. Hughes was married to Sylvia Plath, an American poet, from 1956 until Plath took her own life in 1963.
"Chaucer" is a short poem. You can check it out here. The poem describes someone who gives a loud, expressive speech about Chaucer (a much older and extremely famous writer who lived from 1343-1400) to a group of cows, which is a pretty funny image. The cows start to act as if they're listening, so the person keeps on talking about Chaucer in a passionate way. Eventually there are twenty cows all staring in rapt attention at this person, and the speaker of the poem thinks the whole experience is so bizarre and interesting that he keeps on remembering it afterward. That's when the poem ends.
So are the events in the poem true, and was Hughes writing about his wife, Sylvia? Was she the one who gave the speech to the cows? Maybe so. The poem in question was published in a book called Birthday Letters, which is interpreted as being largely about Hughes's relationship with Plath. It seems likely enough--she was a poet, too, and appreciated literature--but I haven't yet found a good primary source that attests to this fact. We can make a good guess, then, that the poem is about something that Plath really did, but we can't say for sure. In any event, it's probably not true that this poem was recited to cows by Sylvia Plath--it's this poem that's about someone (maybe her) making a speech about literature to cows.
Anyway, the theme of the poem seems to involve praising the old writing style of Chaucer and praising his influence and abilities as a poet, while at the same time, capturing the humor and poignancy of a woman preaching about literature to cows. Notice how the speaker opens the poem by quoting from Chaucer's most famous piece of literature (The Canterbury Tales) and then goes on to talk about how the cows seemed to pay rapt attention and to show that they appreciated Chaucer. The speaker mentions a particular character from Chaucer's work, named "the Wyf of Bath," the discussion of which "enthralls" the cows.