"Mending Wall" by Robert Frost is a poem in which Frost contemplates why he and his friendly neighbor have a wall between their two properties.  He isn't enemies with his neighbor, they don't live in barbaric times where walls are needed to protect one another, they don't have animals that need fenced in...so, why have a wall?  Yet, every spring, he and his neighbor go out to this old stone wall and repair all of the stones that have fallen out of it throughout the year.  The first five lines go as follows:

"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.
The work of hunters is another thing:"

In these lines, Frost is admitting that there is something within himself that doesn't like a wall, and even the earth itself doesn't seem to like a wall.  The earth always seems to push "boulders in the sun," and create huge "gaps" in the wall, so large that "even two can pass abreast" (meaning, two people can walk through the gaps, side by side).  To Frost, even the earth itself seems to try to be shrugging the wall off of herself; the damage caused is so great that he has to go repair it every year.  The last line refers to hunters and the damage they do to the wall.  He goes on to explain that dogs chasing the rabbits chase them right to the wall where the rabbits burrow and hide, and the hunters pull the stones off to help the dogs get the rabbits.

So, the first 5 lines introduce the main concept of the poem, which is that a wall isn't necessarily a good thing, and requires a lot of upkeep--for what good?  I hope that this explanation helped a bit; good luck!