The speaker is taking stock of a place they thought they knew well, and finding it full of things they had missed. The first stanza carries the biggest surprise: deer have been bedding down in six spots just out of sight of the porch, and the speaker never realized. The exclamation mark at the end shows real delight, with a hint of embarrassment at being so oblivious for so long.
The second stanza turns to the decaying outhouse, and here the speaker's question, "how long has the roof been gone?", sounds like gentle self-mockery. Time has been passing, and the speaker hasn't been watching. The last stanza shifts to something more lyrical: shadows of trees on a snowy slope that look like harp strings. The trailing dots suggest the speaker simply stops to look.
The mood is quiet, curious, and a little humbled. Nothing dramatic happens, but the poem suggests that familiar surroundings can still hold wonder if we pay attention.