A part-talkie is a partly, and most often primarily, silent film which includes one or more synchronous sound sequences with audible dialog or singing. During the silent portions lines of dialog are presented as "titles" -- printed text briefly filling the screen—and the soundtrack is used only to supply musical accompaniment and sound effects.
In the case of feature films made in the United States, nearly all such hybrid films date to the 1927-1929 period of transition from "silents" to full-fledged "talkies" with audible dialog throughout. It took about a year and a half for a transition period for American movie houses to move from almost all silent to almost all equipped for sound. In the interim period, studios reacted by improvising four solutions: fast remakes of recent productions, "goat gland" pictures with one or two sound sequences spliced into already finished productions, dual sound and silent versions produced simultaneously, and part-talkies.
The famous so-called "first talking picture", The Jazz Singer (1927), starring Al Jolson, is in fact a part-talkie. It features only about fifteen minutes of singing and talking, interspersed throughout the film, while the rest is a typical silent film with "titles" and only a recorded orchestral accompaniment.
As the financial success of early part-talking feature-length sound films such as The Jazz Singer and The Singing Fool became apparent, producers of silent films which were currently in production, or which had recently been completed but not released, hastened to add or retrofit synchronized dialog sequences so that their films could be advertised as "talking pictures" to a newly sound-hungry public. "You will hear the characters speak from the screen!" the ads could truthfully promise, even if all the audible speech was confined to one brief sequence in an otherwise mute film.
However, some films were hurt rather than helped by such alterations.
The Paul Fejos film Lonesome (1928), an otherwise excellent late silent film, was injected with a gratuitous "talkie" sequence consisting of several minutes of banal small talk between the lead characters. This "goat gland", as such additions were sometimes called, succeeded mainly in causing previously sympathetic audiences to abruptly lower their opinions of the characters' personalities and level of intelligence.