The slow spread, over roughly two to three thousand years, of farming Bantu-speaking communities, later often associated with ironworking from a homeland near the modern Nigeria–Cameroon border across most of central, eastern, and southern Africa — a long, many-streamed process of migration, settlement, and intermarriage, driven by farming and later iron rather than conquest, that seeded a continent with one great family of related languages and laid the human groundwork for most of its later kingdoms, cities, and cultures.
There is no single event here, no battle and no date. Over two or three thousand years, communities speaking related languages spread across most of a continent. They carried farming traditions, adapting crops to new environments, later spreading crops such as the banana in some regions, and in many places becoming associated with ironworking. They did not march; they settled, farmed, married into local groups, and in many places became the dominant population through absorption and displacement. Today several hundred related languages — Swahili, Zulu, Xhosa, Shona, Lingala — are spoken by hundreds of millions of people, all distant cousins of one ancestral tongue. How that happened, and how much of it was people moving versus words and tools spreading, is still genuinely debated.