what does += mean in JavaScript

/* += Add a value to a variable and<br /> store the result as that same variable.<br /> */</p> <p>// The standard syntax:<br /> let a = 2;<br /> a = a + 3; // → 5</p> <p>// The += shorthand:<br /> let b = 7;<br /> b += 3; // → 10</p> <p>/* In JavaScript, the “+” operator can also<br /> concatenate (join together) strings. */</p> <p>// The standard syntax:<br /> let greeting = “Hello,”;<br /> greeting = greeting + ” world”; // → “Hello, world”</p> <p>// The += shorthand:<br /> let farewell = “Goodbye,”;<br /> farewell += ” world”; // → “Goodbye, world”