Dissonance is the spice of music. Like most spices, it tastes bad just by itself, but can be used to add zest and flavor to a recipe.
Simply, dissonance is two or more notes that sound wrong when played together. Play any two adjacent keys on a piano simultaneously and you’ll know what I mean. Here is an example with strings:
Example 1: Dissonance (the spice)
But since music is all about context, the way we present this to our audience makes all the difference in the world. I can dress up the previous dissonant notes so they’ll sound… beautiful. Or I can dress them up to sound scary. It all depends on how I use this spice in my recipe. Listen to this example, which starts with the same dissonance in example 1, and then I fade in some other notes:
Example 2: Music with dissonance (complete recipe)
If I took out the dissonant notes and kept all the “correct” sounding notes, this is what it would sound like:
Example 3: Music without dissonance (recipe without spice)
See how dissonance adds an edge? It gives our sound a bite that just isn’t there in example 3.
By the way, if you like the recipe without spice better than with spice, don’t worry. Mainstream music contains very little dissonance (just like fast-food contains very little spice). The ears of a large percent of the world’s population are not accustomed, and may find it annoying.
The only place where a composer can get away with including lots of dissonance in his/her music and still reach millions of ears (and hearts) worldwide is in film music. Since, like I said, music is all about context, dissonant music will be readily accepted by audiences if it accompanies, say, a battle scene.
It may be interesting to note, though, that in the evolution of things, human taste in general is shifting towards dissonance. If a person from the 18th century had a chance to listen to some mainstream music of today, his reaction would be like yours when you listened to example 1. Our nutritious choices may be leaning towards the healthy side, but when it comes to music, we like to spice things up. Makes you wonder what music will sound like in 200 years, doesn’t it?