# Is there a relationship between the response and predictors?

I have been reading introduction to statistical learning, and I was going through multiple linear regression. This is the topic that I'm reading:

As I was reading further, I encountered an equation that I'm not able to understand. Below is that equation:

It further says that

Please try to explain the above mentioned problem as simply as possible.

Please read https://en.wikipedia.org/wiki/F-test, section Regression problems.