How to get RefererUrl in magento 2 observer?

Make your observer class look like this:

namespace Vendor\Module\Observer;

class ClassNameHere implements \Magento\Framework\Event\ObserverInterface
{
    protected $redirect;
    public function __construct(
        ....
        \Magento\Framework\App\Response\RedirectInterface $redirect,
        ...
    ) {
        ...
        $this->redirect = $redirect;
        ....
    }
}

then you can get the referer url in your execute method like this

$redirectUrl = $this->redirect->getRedirectUrl();

yourObserverFile.php

class ClassNameHere implements \Magento\Framework\Event\ObserverInterface
{
    protected $redirect;
    public function __construct(
        ....
        \Magento\Framework\App\Response\RedirectInterface $redirect,
        ...
    ) {
        ...
        $this->redirect = $redirect;
        ....
    }
   public function execute(\Magento\Framework\Event\Observer $observer)
    {
     return $redirectUrl = $this->redirect->getRefererUrl();
    }
}

Following line will help you to get refferer Url.

$this->redirect->getRefererUrl();