What is the meaning of onsubmit="return false"? (JavaScript, jQuery)

According to the HTML standard, onsubmit is the name of an event handler. The event handler content attribute is treated as the FunctionBody of a JavaScript function which will be used to handle the associated events. In step 5 of "The event handler processing algorithm", it says "Process return value as follows":

• If event is a BeforeUnloadEvent object and event's type is beforeunload
  • In this case, the event handler IDL attribute's type will be OnBeforeUnloadEventHandler, so return value will have been coerced into either null or a DOMString.
  • If return value is not null, then:
    • Set event's canceled flag.
    • If event's returnValue attribute's value is the empty string, then set event's returnValue attribute's value to return value.
• If special error event handling is true
  • If return value is true, then set event's canceled flag.
• Otherwise
  • If return value is false, then set event's canceled flag.

So, in short, return false will cancel the event (or in this case, cancels the form submission).

If you are using a button instead of submit as in my case below:

 <form name="myForm" onSubmit="myFunctionName(); return false">
    <input type="text" name="myText">
    <input type="button" value="Click Me" onclick="myFunctionName()">
 </from>

This is basically done to handle the form submission via JavaScript.

For example - for validation purposes

See the below code and see how it can be beneficial:

<script language="JavaScript">
myFunctionName() {
    if (document.myForm.myText.value == '')
        return false;
        // When it returns false - your form will not submit and will not redirect too
    else
        return true;
     // When it returns true - your form will submit and will redirect
// (actually it's a part of submit) id you have mentioned in action
}
</script>

<form name="myForm" onSubmit="return myFunctionName()">
<input type="text" name="myText">
<input type="submit" value="Click Me">
</form>