A critical appreciation of a poem is a reading that generally considers a poem's meaning, its rhythm and rhyme scheme, its tone, its use of language, and so on. You can discuss all of these elements, or some, or focus in on other things, like the poem's setting or context. Because it's a critical appreciation, it does focus on how you personally appreciate the poem, so your critical appreciation may have a very different focus to someone else's, even when looking at the same poem.
With "Mending Wall," we might start by considering the poem's structure and form. As an earlier Educator has noted, what's particularly interesting is that, while writing a poem about mending a wall and clearing up any gaps that have appeared in it, Frost declines to use stanzas or line breaks. Instead, he writes a poem in blank verse, in a single stanza, with similar line lengths. This creates an interesting block of text of regular size, with little deviation. Literally, it is like a wall running down the middle of the page.
This leads us on to the meaning of the poem. What is Frost trying to say about the wall? We recognize that, even as he builds the wall with his neighbor—so the pair are working together—the wall still seems to represent the distance between them. The neighbor believes that "good fences make good neighbors" or that there should be a firm dividing line between two people and their property. The speaker, on the contrary, builds this wall only because he feels he must. He doesn't know what he is "walling in or walling out," and indeed, the fact that the wall falls down every year seems symbolic to the speaker. He questions whether there might be "elves" or some other power which does not want a wall separating the neighbors. But the neighbor will only repeat his father's statement, refusing to deviate from it: "Good fences make good neighbors."