Bitonality is the use in music of two keys at the same time. Music such as this is called bitonal.
Most traditional music is "in a particular key" e.g. in "C major" or in "D major" or in "D minor". Music that is in C major uses the notes of a C major scale.
In 20th century some composers sometimes wrote music which is in more than one key at once. A well-known example is the beginning of the second tableau of Igor Stravinsky's ballet, Petrushka. The first clarinet plays a melody that uses the notes of the C major chord, while the second clarinet plays a different version of the same melody using the notes of the F sharp major chord.
Mozart used polytonality in his A Musical Joke for comic effect.