"In Your Mind" begins with the speaker posing a question:
The other country, is it anticipated or half-remembered?
This question, along with the title, frames the poem: the speaker's body is in one place, but their mind is somewhere else. The speaker isn't entirely sure if this "other country" they're thinking about is "anticipated," a place the speaker wants to go, or "half-remembered," a place they've already been and are recalling bits and pieces of.
What is clear is that this country is a specific place; not "an other country" but "the other country." The speaker isn't daydreaming about any old location, but one that's important to them. (The phrase "The other country" also just so happens to be the title of the collection in which this poem was published!)
Not incidentally, the caesura created by the comma after "country" forces the reader to pause ever so slightly after this opening phrase. While the sentence could have been arranged differently ("Is the other country anticipated or half-remembered?"), the choice of syntax places greater emphasis on the poem's subject.
And yet, the speaker also isn't sure if this place is entirely real or imagined! Perhaps this is the country of their youth, a place they left so long ago that they aren't even sure that it ever existed.
In any case, the speaker then says that the "language" of this "other country" is "muffled by the rain which falls all afternoon." Notice the use of /f/ and /l/ consonance here, which evokes the soft patter of rain outside the speaker's window. As a constant reminder of the speaker's present circumstances, this makes it more difficult for the speaker to "hear" the other country.
Line 2 is enjambed, propelling the reader into the next line where the poem's setting is revealed:
Its language is muffled by the rain which falls all afternoon
one autumn in England [...]
The speaker is thus daydreaming during a rainy fall afternoon in England (a famously rainy place).
Finally, notice how there's no set meter or rhyme scheme in these lines. The poem is written in free verse: it moves naturally, closely imitating the way people actually think—fitting for a poem that takes place in someone's mind!