You get a pop up message on your pc warning that your computer is frozen due because you’ve been visiting an illegal web site. The warning can be branded so it looks like it’s from the police or other official source. To get your pc unfrozen you need to pay a fine. Except the whole thing is a scam. You may or (more likely) may not get your pc unfrozen by the scammers. You’ll certainly find that your account will be used to pay the ‘fine’ and, in many cases, your credit card details passed on to other criminal gangs.
To avoid being a victim try to keep to known websites and be extremely careful about any files you download.
If your pc is frozen then get it repaired by a reputable computer engineer. And always be ultra cautious about giving out your bank details to anyone online.