Guitar has a way of simplifying some of the complexities of music theory, so while trying to learn those concepts for the purposes of applying them to improve your guitar playing, don't over-think it. You're almost always dealing with something that's more simple then it sounds.
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My goal is to help get you through casual conversation about music theory, so that when somebody brings up a theory-related concept, you can know enough to understand what they're saying to make sense of the conversation and contribute an informed response.
We've already done, pitch, so here we'll cover, melody.
What is "the" Melody?
First, I want to explain this is simple terms, and mostly in my own words. Melody is a collection of tones or notes that are played in succession: Easy enough.
A simple way to think about it, is if you have a song stuck in your head, and you start whistling, whatever you whistle is the melody. So for example, in "Smells Like Teen Spirit" when Kurt Cobain sings the chorus, "With the lights out, it's less dangerous...", that's probably what you would whistle, and would be considered the vocal melody line.
So one way you can identify the melody of a song, is the most prominent vocal line, and not the backup harmony.
If you want to get more specific, any prominent melody line or collection of single notes played by a guitar (or other non-rhythm instrument) is considered melody. So if you're not playing a chord, or a harmony lead part, you're playing a melody line with your guitar.
Melody would include the following elements:
You're the lead guitar player in a band, and the vocalist asks you to "Play the melody line of the chorus during the guitar solo."
What he's probably asking you to do is play a solo that mimics the notes of what he sings during the chorus of the song. So in that situation, he would be singing the melody, and you would follow that up by playing the melody line as a succession of notes on your guitar.
Beyond the Simple
You can get more involved with studying melodies, but for a typical guitar player's purposes, this is all you really need to know.
Check the music theory wiki page for a more thorough explanation if you're interested, otherwise, work on picking the melody lines out of a few of your favorite songs, and you should be good to go.