Transposing (or Transposition) is the process of changing a song or piece of music from one key to another
There are two reasons for transposing into another key:
A.
If the song is too high or too low to sing, the song can be changed into a lower or higher key. Changing the key of a song does not change the sound, pattern or timing of the melody but simply changes how high or how low it is sung.
B.
If the song is hard to play or contains difficult chords you can transpose it to a key with easier chords. E.g., if a song is written in the key of B flat (Bb) major (which many songs are) it would contain chords like Bb, Eb and Cm which may be difficult for a beginning guitarist. If the song is transposed into the key of G major the chords would then be G, C and Am, which are easier for a guitarist to play.