Mrs. Page asks Sir Hugh Evans to test her son William in his Latin grammatical inflections. While Sir Hugh does so, Mistress Quickly repeatedly interrupts with absurd comments and off-color remarks, deriving from unintentional puns on the Latin words which William recites.
This scene has absolutely nothing to do with the plot of the play, and various commentators have suggested that Shakespeare added it as an amusement for an educated audience at a special performance since only they would understand the Latin puns. That is debatable, however, since even the middle class would have had enough schooling (as had William in the play) to catch most of the references. The scene serves at least the technical function of allowing bridging action between Falstaff's resolve to try his luck again with Mrs. Ford, and his arrival at her house. Furthermore, what transpires is amusing in itself (or was, for an audience that understood Latin), as we witness a poor schoolboy caught between a pedant and a daffy woman while he tries to do his lessons. When William hears "focative case" [vocative, in Welsh dialect], he answers "O — 'vocatino, O'" — in all innocence! But Quickly and a sophisticated audience know that "case" also means "pudenda," making the "O" an unintentional, obscene pun.