Above are these former flags along with their present-day iterations. Placing them back-t0-back shows the drastic changes in political structure that they underwent, namely from white-minority rule to universal suffrage and an official policy of representative democracy. What may be less obvious, though, are some elements that have been retained from one to the other.
Most interesting about the transformation from the flag of the former Rhodesia to that of modern-day Zimbabwe is that, despite all of the other changes, the golden Great Zimbabwe Bird remains. The World Encyclopedia of Flags and Heraldry (Slater and Znamierowski 2007: 228) explains, simply, that it represents "the great past of the country." More specifically, since the soapstone artifacts of the ancient site of Great Zimbabwe predate the modern era, it's interesting that both white-minority-ruled (and technically illegal by international consensus since its unilateral declaration of independence from Great Britain in 1965) Rhodesia and black-majority/socialist Zimbabwe have both co-opted the symbol's use in their flags, despite the extreme differences in each administration's politics.
In the context of Roof's use of the former flag, however, its symbolism as the banner of an inherently racist government is most salient. The coat of arms at its center is based on that of Rhodesia's founder, South African mining magnate Cecil Rhodes, also the namesake of the Rhodes Scholarship Programme that allows international students the opportunity to study at Oxford, Rhodes' alma mater. In his name, mostly Anglo settlers originating in South Africa sought to establish a bastion of white rule in south-central Africa.
For starters, the red star of socialism clearly distinguishes Zimbabwe's flag from that of Rhodesia. The green, yellow, and red are common among many African flags and traditionally stand for the earth, mineral resources, and the struggle for freedom, respectively. The black represents the vast majority of the nation's citizens.
But of course South Africa has also wrestled with racial iniquity since its settlement by Dutch Boers ("farmers") in the 17th century. When the British arrived around a century later, the two groups fought vigorously for dominance over this strategic and vast territory that had been populated by various groups of black Africans for centuries, if not millennia. The former South African flag, in currency from 1928 until apartheid was dismantled in 1994, represents that jockeying for power between the descendents of Dutch and British settlers. The orange-white-blue horizontal tricolor of the flag's main body hearkens back to William of Orange, the Dutch nobleman who rebelled against Spain and gained the Netherlands its independence in the 17th century, around the same time that Dutchmen began to colonize South Africa.
Looking at the old South African flag's central charge, the complexity of history and politics manifests itself in full force: the Union Jack shows the impact of Great Britain, particularly in the case of its former Cape Colony; the central flag is that of the Boers' Orange Free State, which took the orange of William's prinsenvlag for its three horizontal pallets (bars) on a white field, along with a canton (corner) of the alternate Dutch tricolor of red, white, and blue, the very same that remains in currency to this day for the Kingdom of the Netherlands. On the right is the flag of the other independent Boer republic of the late 19th and early 20th centuries - the Transvaal. Its only distinction from the flag of the Netherlands proper is its green field at the hoist (left side), representing the open expanses of the territory's veld.
When Nelson Mandela won the presidency at the nation's first fully democratic elections of 1994, it was only fitting that a new flag represent such a sea change. Noted for its unique design - which is also wildly popular among South Africans of all walks of life - the pall, or Y-shape, according to flag expert Whitney Smith, "stands for the coming together of many parts and the merging of past and present" (2001: 86). The black, yellow, and green represent Mandela's party, the African National Congress (ANC) while the red, white, and blue stand for the flags of the erstwhile Boer republics mentioned above.