An interval is the space between two notes. If you want to picture two fingers landing fairly close to each other on a piano, you understand what an interval is. And intervals have a history, and, of course, they have what makes music interesting, they have emotions.
Historically, the fifth was one of the first interval to be explored in western music. Some instruments couldn’t be played without having a fifth going throughout the tune. Bag pipes are like that. No matter what, you have that drone going, it’s almost a metallic interval. Some say the fifth is empty. Maybe but it’s functional.
After that came the third, the major, the minor. The major one was supposed to be for celebrations and happy moments (But there are many exceptions), and the minor was for the blues and sorrow (but there are also many exceptions to that). You can add those to a fifth, it’ll give you a chord, a triad (triad is for the number “three”, like 3 notes). So the 3rd brought the major chords and the minor chord. Even if you don’t know what I’m going on about, you’ve heard those a million times. It’s all over music. That musical device has been living large for a long time.
With the 3rd came the sixth, their little brothers, also major and minor. They don’t have as much significance, but it’s pretty close. A sixth is a third upside down. Their quality are upside down too. A major third like A to C becomes a minor sixth when C to A. Funny little things. Just like the irregular verbs. You know, knew, known.
Then came the seventh. This one is more elaborate than the ones I just talked about, more explosive too. Handle with care. A minor seventh can be combine to all sorts of triad: Major, minor, diminished, etc. They have their role too. They make a rich harmony, a denser texture to the music. The Christmas chord is one that uses a seventh it’s the minor 7 flat 5 or m7b5. Made Mariah Carey a fortune. I won’t mention them all but I will say that the blues functions very well with dominant 7 chords and that jazz is a type of music that explored the seventh chords quite extensively.
Then there are the seconds. Very explosive. Keep it sealed when not in use. The major second is used in rock in sus 2 chords, for example. That allows the music to be rich but stay neutral since there are no third to tell you to be sad or happy. And then there’s the minor second. It’s a dissonant interval. For a long time nobody used it but for some really great masters at harmony like Johann Sebastian Bach. Even today it’s a difficult one to place correctly in the music, like handling nitroglycerine (Never seen any, but heard some bad stuff about it).
Then there is the fourth. It’s a reversed fifth. Get it?
Then there is the flat five or tritone. This one here has a bad reputation. Religious people don’t like it. It’s called the devil’s interval. Ray Charles got in trouble with this one. It is not a pretty sound, but sometimes you need it to describe the world.
If you have no clue what this all means, you can take piano lessons and find out (I know an excellent teacher).