Imagine if you love playing with Legos, but one day you find out that some of the pieces don't fit together and don't work with the others. This is what happens with software, specifically Java software, when there is a problem with something called jars.
Jars stand for Java ARchive and contain a bunch of pieces of code that software needs to run. Different jars can have overlapping pieces of code, which means they might not work together or mess things up in the software. This is called jar hell.
It's like having a bunch of puzzle pieces from different puzzles, but they all look similar, so you accidentally try to put them together even though they don't actually fit.
Jar hell can be a big problem for developers who need to make sure all the jars in their software work together. It can also be a problem for users who want to use a certain software but can't because it's stuck in jar hell.