I've never heard the expression chord-melody. Im imagining you mean something like Stanley Jordan doing eleanor rigby?
Its the process of combining the melody with the chords of a song. This is very common in certain kinds of music like jazz.
If you play a piano, you use your left hand to play chords and the right hand to play the melody. You can do this simultaneously, or sometimes more or less simultaneously because 2 hands (and five fingers on each hand) are available for this process. This is a little oversimplified, but more or less, this is generally true.
On a guitar, you only have 4 fingers on one hand to play something (either single notes or chords). On a few occasions, you might use your thumb to play a bass note, but I’ll just keep it simple.
So merging the melody together with the chords requires some engineering. One rule is that the melody is always the highest (from a frequency/pitch perspective) note. Another rule is that often the first beat in a measure is where you play the chord. But, here again, I am oversimplifying. There are times you have to do what makes sense versus following rigid rules.
Jazz guitarists do this quite a lot, and I suppose classical guitar as well, although my experience in this genre of music is limited.
When you have a band with multiple instruments, or a singer, you can have one instrument or the singer handle the melody while other instruments play chords (well except for the drummer & bass player of course). So, with a lot of genres of music, you don’t need to do a chord-melody. You can, but its not necessary.
However, if you have the patience and skill, when you successfully do a chord-melody, it is the most complete picture of the song that can be presented by one single guitarist.
You can have very skilled musicians in any kind of music. The genre doesn’t determine or limit the skill. But, with jazz, there are some additional elements or layers that you must pay attention to.
Maybe that’s why a lot of musicians steer away from playing jazz. Those additional elements make it more complicated. Another complication is that for a jazz solo, the key often changes. In a song I just played, there were 5 keys: A major, C major, Bb major, D major, and A harmonic minor. And the key kept shifting amongst those 5 keys depending on what section of the song was being played. It was actually a relatively simple song relative to others I’ve played.
Another key difference is the chords being used. Most other forms of music (rock, blues, folk) use triadic chords (three notes). So, the chords are mostly major or minor chord, with maybe a dominant 7.
In jazz guitar, it's typically 4 note chords. The basic building blocks are major 7th, minor 7th an dominant 7th. But, there are hundreds of different chords a jazz guitarist (or any other instrument) will use. You’ll see 6th, 9th, 11th, 13th, flat/sharp 5 (with a minor or major 7th in the prefix), flat/sharp 9 (with a minor or major 7th in the prefix), half diminished, diminished, suspended chords, augmented chords, and many others (too numerous to list).
BTW, I took a real quick listen to that Stanley Jordan song. He was playing single notes mostly. He’s a skilled musician though no doubt.
