Hello, recently a lot of websites started to use Web Miners on their websites that uses their visiters CPU power to mine Monero while they are browsing the website.
So Avast Antivirus started to block those Web-Miners so your PC won't freeze or you CPU won't overheat while browsing websites :)
If you don't have any antivirus on your computer I would really suggest you to get Avast Free Antivirus to prevent Web-Miners such as this one.
Have a good day!