Transposition is moving pitches up or down by some interval. All pitches are raised or lowered the same number of half-steps, so melodic contour is preserved and all that has really changed is the "key" or pitch.
There are several transposition options that allow you to do such things as transpose entire scores into a new keys, transpose individual parts for part extractions, work in transposed scores, and transpose sections of music.
You can change the transposition by clicking on one of the three transposition items in the main tab "Home":
Click on a topic for more information: