With Karl Marx having shuffled off his mortal coil, his collaborator and donor Friedrich Engels reviews some basic info about the Manifesto's publication.
The Communist League in 1847 asked Marx (and, Engels misleadingly adds, Engels himself, too) to write a public declaration and explanation of the secret organization's goals—in other words, a manifesto. Karl wrote it in German; it was published in early 1848. The Manifesto was soon translated into several languages.
But the workers in the June uprising of France's 1848 revolution, on whom Marx had pinned some seriously high hopes, were defeated; several communists of the League were hunted down, arrested, and imprisoned; and the League itself was shut down. It seemed the Manifesto was doomed to be just another forgotten work.
Plot twist! The International Working Men's Association (sometimes called the First International) arose a decade and a half later in an attempt to organize militant workers with different political views. According to the preface, this organization's struggles were matched by a steady increase in acceptance of the Manifesto.
Basically, it looks like everyone is finally agreeing with Karl.
Well, the Manifesto surely got read and translated a bunch more. The preface lists multiple editions—a Russian translation in 1863, a Spanish version in 1886, and plenty of others—and states that the history of the Manifesto's spread reflects the history of the working class movement itself. That's pretty convenient….
Engels distinguishes between socialism and communism and makes it clear that the Manifesto is about the latter. Socialism is a middle class thing, he explains, and you can debate it in what he sarcastically calls respectable company—in other words, among middle- or upper-class folks who want to keep their positions. But communism scares such people: it's a working-class beast, and it aims to change the entire social system completely by ending all class differences forever.
Engels says the Manifesto was his and Marx's joint production, which is actually true. It's just that the League commissioned Karl only, and now most sources, including us, only give the famous guy's name as the author.
Engels goes on to say that the fundamental idea was Karl's. Then we get a handy paragraph summarizing that basic point.
The basic gist of the Manifesto, Engels says, is that political and intellectual ideas—and therefore all of history—are based on the economic system of the day, and thus on class struggles. Today, he says, the exploited and oppressed proletariat can only free themselves by both overthrowing the rich bourgeoisie and ending economic class altogether.
Engels says he believes the Manifesto's key concept will become as important to the subject of history as Darwin's theory of evolution is to the subject of biology.
The preface states that the Manifesto is still correct in general. It says the tract never emphasized the practical steps at the end of Section 2 about how a vanguard state (a temporary form of government) should transition a society to communism, and those steps would have to be different in whatever particular countries become communist in the future.
Also, the critique of socialist literature in Section 3 as well as the comparison of communists (actually the Communist League) to various political parties in Section 4 are out of date, being specific to the time the Manifesto was written.
But since the Manifesto is now a famous document, it should be left unchanged, Engels says. Don't fix it if it ain't broken.
Engels notes this 1888 translation into English was a joint work of himself and Samuel Moore, with a few explanatory footnotes by Engels. That's why it's considered the authorized translation.
Note: Other prefaces later written by Engels tend to follow the same lines as this preface, but they talk about the further spread of Manifesto translations as well as the growing working class movement.