Operation high roller was a very bad thing that happened on computers a few years ago. Bad people would use secret tricks to get into other people's bank accounts and take their money. These bad people were like sneaky thieves, but they weren't using guns or masks. They used the internet and special computer programs to steal lots and lots of money, sometimes even millions of dollars at once! They would pretend to be someone else and trick the bank's computers into giving them the money.
Now, the people who were stealing the money were not using their own computers or sending emails like we do. They were like masterminds, with lots of money and smart brains that they used to create a special plan. This plan was called Operation High Roller. They would take control of hundreds, maybe even thousands of computers around the world which belonged to ordinary people like us. These people didn't even know their computer was being used in this sneaky plan!
The bad people would secretly take over some parts of those computers, so they could control them from far away. They would make the computer do things it wasn't supposed to do, like sending fake information to banks, and that's how they stole the money.
Luckily, the police found out about this bad plan and worked together with banks all over the world to stop the bad guys. They caught many of these sneaky thieves and made sure they could not steal any more money. Now, banks use much better security to protect our money and keep us safe. We can keep using our computers and phones to look after our money, but we always have to be careful and use safe passwords, so bad people cannot steal our money!