What is Correlation Coefficient?

So, now that I've ran some data in SPSS, the thing that I'm still quite confused is the Correlation Coefficient...

found this on youtube:

Regression Analysis & Multiple Measures

I'm trying to understand the use of these 2 analysis.

I found this website helpful..

http://www.microbiologybytes.com/maths/spss4.html

Then, to understand the steps further, I looked into youtube to find a similar explanations...

http://www.psychstat.missouristate.edu/introbook/sbk27.htm