Wikipedia
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.