In "Mending Wall," Frost writes a contemplative poem based on the activity of going out with his neighbor each spring to mend the stone wall that divides their property. Frost himself doesn't really like the wall--he feels it is unnecessary, unfriendly, outdated, and a bit rude to have. However, his neighbor, who seems to be steeped in tradition, says, "Good fences make good neighbors." This is an old saying that seems to imply that you can be better neighbors if there are boundaries; that way, you don't end up fighting over what property is whose. It helps create lines, which eliminates potential conflict. So, for example, if there is a tree that is kind-of in-between two houses, who has to rake the leaves every fall? Without a fence, neighbors might argue about this issue, or just silently seethe with rage as the leaves build up and the neighbor doesn't rake them. With a fence, the tree definitely belongs to a certain person; they are responsible, and the potential problem is solved.
So, that is what the quote means. Frost, however, disagrees. He gives several reasons for this in his poem, but the main reason is that their properties don't really need them. He has apple trees, the neighbor has "all pine," and, as Frost says,
"My apple trees will never get across and eat all the cones under his pines."
Trees don't need to be fenced in or out--they don't steal or interfere with anyone, like someone's dog would, if unchained or fenced. He also mentions that sometimes fences are put up to keep the cows in, but, "here there are no cows." They don't have animals to keep in or out, and no property disputes. He also asserts that there is something ominous and unkind in a wall--he says that a wall implies you are keeping something dangerous away, or dangerous in, and that's not very pleasant. In the end, he even compares his wall-loving neighbor to "an old stone-savage," symbolically indicating that keeping walls is a rather savage ritual that is only needed in more dangerous times.
I hope that those thoughts helped; good luck!