One of the things that sets a great singer/songwriter aside from an average one, is his/her understanding and application of the fundamentals of music theory. In simple terms, music theory is an understanding of the language of music and how music works. The more you know about music theory, the better you will be able to express your emotions, ideas and thoughts with music - whether it be through singing, dee-jaying or songwriting. 

As an artist, a basic grasp of music theory will help you understand how songs are composed, which will improve the overall execution/delivery of your song whether it be through your ability to improvise on a song, putting your own spin/creative touch on it, or your ability to transpose a song to better suit your vocal abilities. 

You don’t have to have “perfect pitch”, but a working knowledge of musical scales will go a long way. You must first understand this concept: Music is a Language, and no matter the language, they all have one basic component in common - the alphabet. In the case of music, the alphabet is the scales. Simply put, a scale is a number of musical notes that have been grouped together. An example of a major scale that you’re probably already familiar with is “Do, Re, Me, Fa, So, La, Ti, Do”. Knowing the different scales and the difference between minor and major keys will prove very useful. Here’s a good place to start → http://study.com/academy/lesson/how-to-read-notes-on-the-staff-treble-cl...