Modes were used to organize the melodic and harmonic elements of music in the
Middle Ages and Renaissance periods. You can think of modes as scales which
use the white keys of the piano. The Ionian mode (same as the major scale) uses
the white keys from C to C. The six remaining modes also use the white keys
but start on different notes. For example the Dorian mode uses the white keys
from D to D.
During the 17th to the late 19th Century, the modes were not commonly used,
being replaced by the major and minor scales. Modes are often heard in music
today.