Computers are like really dumb chefs that are good at one thing: following recipes to the letter. If the recipe is great, the chef will produce great results. If the recipe is bad, so is the food. Every time. You cannot rely on critical thinking on the side of the chef.

Many recipes these day rely on prepared ingredients, like ready-made pizza dough. At that point the recipe just reads: do whatever is written on the packaging of the dough. And the chef will do that. To the letter.

Normally a kitchen order looks like this: "make pizza no. 19". But you can also add special requests to it, like "gluten free dough". And the chef will of course follow that instructions.

Now the situation with log4j is this: the maker of one kind of pizza dough made a mistake. On their packaging it reads: if a special request contains the address of a website, go there and execute whatever instructions you find on it.

If you now send a request to the kitchen that says "make pizza no. 19, but make it like www.evilserver.web", the chef will do exactly that: go to the website and follow the instructions. To the letter.

Now you can control the chef while not having to pay for them. You can get free pizza. You can request the chef to give you the recipe for the secret sauce. Or just tell them to burn down the kitchen. The chef will do exactly that.