Watch these videos.
1.
Dynamics: Everything You Need To Know in 7 Minutes(as well as how the piano got its name)
2.
What are Dynamics in Music? | Music Theory for Kids | Building Blocks of Music | Green Bean's Music
3.
Please note: this is how crescendo and decrescendo are spelt. There are spelling mistakes in the video.
Dynamics example
4.
Sharps Flats & Naturals Song
5.
Dynamics in Music Explained: Loud or Soft?
6.
Music Theory for Beginners: Dynamics
Accidentals, the basics – Music Theory Crash Course