An avalanche is like a big snow slide that happens really fast and can cause a lot of damage. In the same way, a phishing group called Avalanche is like a sneaky group of bad guys who work together to steal important information from people on the internet.
These bad guys might send you an email or a message that looks like it's from a trusted source, like your bank or your boss. But when you click on the link or enter your information, they can then steal that information and use it to do bad things, like take your money or pretend to be you on the internet.
The Avalanche group was especially sneaky because they had so many different ways of tricking people into giving away their personal information. They could make fake websites that looked just like real ones, or send out mass emails that looked like they were from official sources.
Thankfully, authorities worked hard to stop the Avalanche group and shut down their operations. But it's important to always be careful when it comes to your personal information online, and to never give it away unless you're completely sure it's safe!