RandomX is a Proof Of Work algorithm developed by Monero contributors and adopted by Monero from release 0.15.
This innovative POW is optimized for CPUs and it's based on execution of random code and other memory-heavy techniques. The goal is to discourage the use of specialized hardware (like ASICs) for mining Monero. RandomX was created with the intent of keeping mining decentralized and to create a more egalitarian distribution of the block rewards.
RandomX specifications and development can be found on the GitHub repository, while an article dedicated to RandomX made by the Outreach workgroup contains more information about the history of RandomX and its relation with Monero.