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.
In the poem "Mending Wall" by Robert Frost, we also get the impression that the author is talking about subjects other than Nature alone. With the words "Something there is..." he may be saying that the something is something in human nature that doesn't like or need walls either, something that doesn't like or see a need for barriers or conventions or restricitions all the time. Robert Frost could be talking about himself here too. Yes, he goes on to say - lots of people do believe that good fences make good neighbours because no-one then bothers or encroaches upon anyone else. But others (Frost himself?) believe that there is no need,for example, to protect cattle from apple trees, or corn from forestry plantations. He is saying that some people use protection as an excuse for peoperty delineation and staking claims. Think of the Berlin Wall/Great wall too.