JS var addTogether function variables?

function addTogether(oneThing, anotherThing) {

var newThing = oneThing + anotherThing

alert(oneThing + " + " + anotherThing + " = " + newThing)


How can the parameters be changed?
What is the purpose of the “+” (middle plus) and the “=”?
Why do we need to write function first?
What exactly is the variable?

The best way to find out is to experiment. If this is the beginning of your coding journey, I would very much suggest you to experiment a lot. Then read (or use your preferred medium), and then experiment more.
Now, regarding your question: you are the one who has the power to change parameters or manipulate them in any way you please.
We write functions to reuse code and make it more readable. This way you can call a function as many times as you wish. Functions are very versatile and powerful in JS. Invest time in learning them.
The purpose of those operators and operands is very well explained in the exercise.
A variable is a named location in memory. It points to that piece of memory where something that you assign to that variable is being stored for a period of time. It can be empty, a string, a number, an array, a function, anything that you can store.