As my JS is basic maybe my basic explanation may help 
(or not)
You are calling the function bake() and passing 350 as a parameter. i.e. bake(350).
Then when the function runs the degrees (argument) will equal the 350 you sent it. That will then set the value of message to “That’s a very comfortable temperature for me.” (because degrees are greater than 100).
You are assigning the value of bake(350) to a variable called status. The variable status therefore contains the value of the message variable that you returned. If you were to write out the value of status you would see the value of your message.
e.g.
var status = bake(350);
console.log(status);
// will show "That's a very comfortable temperature for me."
status = bake(50);
console.log(status);
// Will show "I'm not a refrigerator!"
status = bake(600);
console.log(status);
// Will show "I'm not a nuclear reactor!""
If you run that with your code you will get the message displayed three times as shown above.
From the above you can see that you can call the function bake with various values and receive the appropriate answer in the variable called status.