Basically it involved parsing JNDI stuff which involved grabbing remote code (but that was a niche feature of JNDI in the Dev’s defense). Basically, you may think it is just something like variable substitution but can involve much crazier stuff

https://theconversation.com/what-is-log4j-a-cybersecurity-expert-explains-the-latest-internet-vulnerability-how-bad-it-is-and-whats-at-stake-173896

this is cool