The most obviously important symbol in this story is the wall which the mother, father and grandmother had all contributed to improving. To understand its significance and context, one needs to scrutinise the story's setting.
The story is set during the apartheid era in South Africa's troubled past, where prejudice and discrimination between whites and other races were the order of the day.
One can readily assume that when the family moved into their comfortable suburban house, there already existed a wall, to demarcate not only the property, but to create boundaries and ensure that unwanted visitors were kept out and the residents were secure. In this sense, the wall represents the clear demarcation between the races, more specifically, the separation between whites and other groups. This was a reality even before the apartheid regime came to power, although the lines were slightly blurred.
When the white Nationalist government came into power, apartheid became official. Laws were promulgated to separate the races into distinct groups. The whites, as a consequence, were given the fat of the land. They were granted extensive privileges whilst other groups, especially black South Africans, had to make do with the crumbs. There was much prejudice and whites deemed themselves superior and because of this, they thought it necessary to protect themselves against embittered groups who would fight tooth and nail to gain equal status. More laws were created to ensure greater security for the privileged whites and the police (who was, by definition then, dominated by whites), were granted extensive powers - South Africa, in essence, became a police state.
The family raised the wall because of the increased risk:
You are right, said the wife, then the wall should be higher. And the wise old witch, the husband's mother, paid for the extra bricks as her Christmas present to her son and his wife.
In this regard, the increase in the wall's height, represents the South African government's attempts to create further division, by enacting more stringent laws, such as for example, The Reservation of Separate Amenities Act, The Prohibition of Mixed Marriages Act, and many others. The government created so-called bantustans to widen the gaps even further. The more separated people were, it was believed, the more secure the privileged class would be.
However, the protests against the government's extreme prejudice and unjust laws grew. Disenfranchised residents more and more declared their resentment and openly challenged government. States of emergency were declared. These developments are reflected in the story through the fact that the family decides to further increase security by adding serrated stainless steel wire coils to the top of their wall. What they do not realise, however, is that the danger does not lie on the outside, but within. The story's tragic ending makes this pertinently clear.
The story makes the point that creating divisions does more harm than good, that the best attempts at keeping people apart will eventually culminate in failure, since we all, as social beings, innately belong together and will intuitively seek one another's company. It is a natural fact of life.