“Internet sites are using YOUR computer without your consent to make money - i.e. they are using you property without permission and you get no compensation.”
The question is HOW? I know what cryptocurrency is but how do they “mine” it? If you have Bitcoin are they transferring a portion of it value to their computer?
To “mine” - you need cpu power (or gpu) - in other words, the site you visit secretly lets your computer do some work computing the next “coin” - if enough ppl visit the site, there are then hundreds of computers all working for the site. It costs them nothing to let this run on your computer but they get all of the profit if the “mine” a new coin.
As to “mining” - it is actually rather complicated, but, in a nutshell, a unique value needs to be crpytographically computed which then becomes the coin. When these things start, a coin can be mined in a matter of minutes on a simply computer. After a while it gets harder and harder to comute the value and entire server clusters are used and it can take days or weeks.
So, if a site can “borrow” your CPU to take care of some of the load, they have more power to compute without buying more hardware.