what's the difference between echo and return??

Return spit out the product of function and anticipates function end.

Echo prints out value to your web page.

<?php
    function sayHi() {
        return "Hello";
    }
    sayHi(); //Run sayHi() function. Do no more than that.

    $hi = sayHi(); //Assign whatever that is returned by sayHi() to $hi variable.

    echo sayHi(); //Print out whatever that is returned by sayHi();

As noted by Fabio and Gunhoo, echo displays to the screen. Return is very different.

When you have a function (or method in OOP), the code executes within the function but does not send anything out unless you have a return. The return itself does not display anywhere. You can assign it to a variable elsewhere for more to be done to it, you can echo it, or you can just ignore it. For example, you could do this with your function:

<?php

function value() {
  $number = 1;
  return $number;
}

$val = value();
if ($val !== 1) {
  echo "The number is not 1.";
} elseif ($val === 1) {
  echo "The number is 1.";
} else {
  echo "There was an error.  The number is $val.";
}

As you get more into functions you will learn to pass values into the function that will make my example a little more meaningful. But you can see how using return allows you to then analyze the result of the function with more code. When you use databases, you often return an array of values derived from the database. You then need code to process the array into something you can use for the site.

echo prints the value as strings and return calls, for instance a function, and store the value for future use.