DLL hell is when your computer has a bunch of little programs (called DLLs) that it needs to run other programs, but some of them are old or broken and can't work together. It's like trying to build a Lego tower with some blocks that don't fit together properly.
For example, let's say you have two programs that need the same DLL, but one needs an older version and the other needs a newer version. If you install the newer version, the program that needs the older version won't work anymore. But if you install the older version, the program that needs the newer version won't work. This is a big problem for your computer because it's like it has to choose which program to make happy and which one to make sad.
It's called DLL hell because it's a really frustrating problem that can be hard to fix. It can make your computer crash or make programs not work at all. It's like being trapped in a maze with no way out!
To avoid DLL hell, software developers have to be very careful when they create programs and make sure they use the right versions of DLLs. And if you're using an older program, you might have to stick with an older version of Windows or a certain version of a program to avoid DLL conflicts.