Sevenths

What is a seventh, i understand for scale and all, but for chords. What is it? Why does it work, etc

The seventh is the seventh note in a scale. We make chords from scales.

e.g. Key of C: C D E F G A B

C Major:

1. C major
2. D dorian
3. E phrygian
4. F lydian
5. G mixolydian
6. A minor
7. B locrian

C major chord - C E G - this is 1, 3, 5 of this scale

C Major 7 - C E G B - this is 1, 3, 5 and 7 of this scale.

Does that make more sense?

I figured thats what it was, but when i went to test my theory it didn't add up. Thank you.