What are dynamics?  Dynamics tell us how loud or soft to play or sing music.
Why are they important?  If all music was the same dynamic, it would all sound the same and be boring to listen to or perform!
Why do dynamics have Italian names?  Italian is the language of music.  Most music vocabulary has come from Italy.
The different dynamics and their abbreviations:
Crescendo: Tells the musician that music written above the crescendo gets gradually louder
 Decrescendo:  Tells the musician that the music written above the decrescendo gets gradually softer
All crescendos and decrescendos must have a starting dynamic and an ending dynamic, so that the musician knows how loud or soft to start, and how loud or soft to end