Alright, let's elaborate with some simplified explanation about the Scanner class.

It is a standard Oracle class which you can use by calling the import java.util.Scanner.

So let's make a basic example of the class:

class Scanner {
   InputStream source;

   Scanner(InputStream src) {
       this.source = src;
   }

   int nextInt() {
       int nextInteger;
       //Scans the next token of the input as an int from the source.
       return nextInteger;
   }
}

Now when you call Scanner input = new Scanner(System.in); you make a new object of the Scanner class (so you make a new "Scanner") and you store it in the variable input. At the same time you are calling the (so called) constructor of the class, with the parameter System.in. That means it is going to read from the standard input stream of the program.

Now when you are calling input.nextInt(); you execute the method from the object you just created (also documented). But as we see, this method returns a integer, so if we want to use that integer, we have to assign the call to a variable like you do:

int i = input.nextInt();