The erosion of the fence and the apple trees are personified.
Personification is the description of something innate or not human as if it were a person. For example, there is some phantom force personified in this poem that doesn’t like walls and tears this one down every year.
Something there is that doesn’t love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun;
And makes gaps even two can pass abreast.
There is not actually anything there that doesn’t like walls. Time erodes the wall and causes parts of it to come down. The speaker just personifies this force of normal erosion, saying that it “doesn’t love a wall.” The one who really does not love the wall is the speaker. He gets annoyed by the process of repeatedly rebuilding the wall, which he considers to be unnecessary in the first place.
Another example of personification is the apple, trees, which may actually be compared to animals.
My apple trees will never get across
And eat the cones under his pines, I tell him.
He only says, ‘Good fences make good neighbors.'
The apple trees are not actually able to move, and of course that is the speaker’s point. Apple trees getting up and walking over to eat pinecones is very silly. This is the reason why the speaker doesn’t want a fence. It makes sense to have a fence if you have animals that you have to keep in.
The moral of the story with this poem is that you are going to be able to more easily get along with your neighbor if there is a fence between the two of you. The speaker prefers closer contact with the neighbor, but the neighbor wants to maintain the fence between them. The less you see of your neighbor, the better, according to him.