This is a measure against bots and malicious requests similar to a CAPTCHA. However, instead of having to do work yourself, your browser is given a calculation task that it has to solve to ensure that it is a valid client. This concept is called <a href="https://en.wikipedia.org/wiki/Proof_of_work">Proof of Work</a>. The task is calculated in a few seconds and you are granted access to the website. Thank you for your understanding and patience.