Question

Where does GFP come from? How does it help researchers know if a gene is active?

Answer 1

GFP is known as a reporter gene in biochemistry, biotech, etc. Let us pretend you found a new promoter in mouse, and you want to know where it is being expressed.
Basically what you do is connect your promoter of interest to the GFP cDNA. Linearize the whole thing, then micro-inject into a fertilized mouse egg.
The mouse will grow up with this 'transgene' inside it. Whenever that promoter gets turned on, the tissue where it is being expressed will glow green.

This is a very basic application, but it demonstrates how GFP is used. From there, you can figure out what tissue your promoter works in, and then start to do further experiments on it to determine its function.