In the commentary and names of music you will hear of key signatures like C-Major and D-Minor. Understanding a bit about this surprisingly technical subject will deepen your appreciation of what you are hearing. Here some videos on YouTube that should help. These videos are all by Michael New.
1. Major and Minor Keys (Music Theory)
2. How Basic Chords Work - Music Theory Lesson 1
3. The Circle of Fifths - How to Actually Use It
And here is the Circle of Fifths referred to in the last video.