The poem's title gives readers context for these opening lines: some men are playing "football" (what some countries call soccer) in or near Slack, a village in England.
The poem itself then opens right in the middle of the action: this game takes place "Between plunging valleys, on a bareback of hill" (i.e., a hill with no trees). Referring to this place as the "bareback of hill" subtly personifies the landscape, bringing the natural world to life around the players.
The players, meanwhile, are dressed "in bunting colors." "Bunting" can refer to a small, colorful songbird or to festive, decorative flags; as an adjective, it means that the men are boldly dressed (perhaps in the colors of the British flag).
This is a joyful, energetic game: both these men and the ball they're kicking around are bouncing around the field. Check out the use of diacope/epistrophe in line 3:
Bounced, and their blown ball bounced.
The repetition of "bounced" highlights the relationship between the men and their ball. On the one hand, the men "bounce" simply in the sense that they jump around after their ball. But this line also implies that the men, like the "blown ball," are being "bounced"—that they, too, are being "blown" about by the wind. Both men and ball are playthings for the natural world.
The poem is written in free verse, meaning it does not use a regular meter or rhyme scheme. Instead, Hughes uses sonic devices to evoke the energetic rhythms of this game. Listen, for example, to the striking /b/ alliteration of these lines, which adds an appropriately bouncy rhythm to the poem's first stanza:
Between plunging valleys, on a bareback of hill
Men in bunting colours
Bounced, and their blown ball bounced.
Note, too, how the first two lines here are both enjambed. As a result, the poem moves swiftly down the page, subtly mirroring the men's playful energy. The stanza then comes to rest at a reflective full stop in line 3.