Credit to my music theory teacher, Robert Peri, for this information
Not much is known about Guido personally, largely because he lived from 995 to approximately 1050. However, Guido was extremely important to modern music.
Guido's experience with music came from the fact that he was a Benedictine monk. Benedictine monks are well known for their chanting, which is still done today. But back in Guido's day, there was not consistent musical notation, nor any truly consistent musical ideas
Guido's primary problem was trying to teach his monks new chants he had written, and even harder, sending a monk to distant monasteries to communicate his new chants.
To solve the first problem, he invented two things:
The Guidonian Hand and
The Solfeggio
The Guidonian hand was a device used by Guido to teach his monks the pitch of the notes in his chants. Hold your hand so that you palm is facing away from you and your fingers are horizontal. To indicate the pitch of a note, you point with your other hand to a vertical position on your hand. Notice that your four fingers (excluding the thumb) act as place markers. From this method of teaching, a written notation, the musical staff, soon arose. Guido's staff had four lines and was a direct predecessor of the one we have now.
The Solfeggio is also very important. This is the name given to the syllables "do re mi fa sol la ti do". Guido invented this, but at the time he made it, the syllables were "ut re mi fa sol la". At that time, the seventh of the scale was not generally used. In addtion, you may wonder why "ut" was used instead of "do." The answer is that these syllables come from the first syllable of latin phrases in a particular chant. These phrases are: "Ut queant laxis," "Resonare fibris," "Mira gestorum," "Famuli tuorum," "Solve polluti," and "Labii reatum."
I do not know why ut was changed to do, but that is why it was originally ut. Although Guido often gets the short shrift in musical history, it is impossible to dispute that he began the development of tools that would allow composers like Bach and Beethoven to contribute their enormous works
Guido's major work was Micrologus de disciplina artis musicae written in 1025. This described the state of music theory at the time and presented some of his work in music theory.
To read about the next major development in musical notation, see Franco of Cologne.