In "Mending Wall," what does the first line mean: "Something there is that doesnt love a wall that sends the frozen-ground-swell under it."

Literally, what this means is that (the speaker says)
nature does not like walls.  He is saying that nature does not like to be hemmed
in.

Because nature does not like walls, he says, it tries
to break them down.  The frozen ground swell is probably what is called a frost heave. 
It's a thing where ground (water in the ground, actually) freezes and thaws and swells
up because of that.  If it does that under a fence, it can break the fence.  You can see
it happen to roads in places that have the right climate.