That Rhythm Thing is a Javascript program running in a VCV PROTOTYPE module. To get a feel for what it does, look for videos on YouTube that show how Steve Reich’s 1972 piece, Clapping Music, works. In short, you have two tracks playing a rhythm in a loop. Each track loops for a set number of repetitions and then one of the tracks jumps ahead by one step and the cycle continues. In Reich’s piece, both “tracks” are playing an identical pattern and a cycle lasts 8 repetitions at which point one of the tracks starts looping from step 2 of the pattern while the other continues to start on step 1 ...
That Rhythm Thing works in a similar way. Track 1 always starts its loop at step 1 of its pattern but Track 2 moves its starting point up by 1 step each time a new cycle starts. Unlike with Reich’s piece, each Track can play a different rhythm and the number of repetitions per cycle can be changed. You can change the number of repeats or select different patterns for either Track at any time. You can also edit individual steps in a pattern, all while the module is active.
The PROTOTYPE Module runs "raw" code (Javascript, in this case). It is prudent, therefore, that you trust who you are getting the code from (including code that has been saved in a patch that contains an enabled PROTOTYPE module). The VCV PROTOTYPE will caution you about this when you first load a patch containing a PROTOTYPE module. The PROTOTYPE Manual (https://vcvrack.com/Prototype#manual) puts it this way: "When opening a patch containing a VCV Prototype script, a security warning is displayed before it runs. Only load scripts from patch artists you trust. Running Prototype scripts from untrusted sources may compromise your computer and personal information."
I'm not sure what you need from me to qualify as a "patch artist you trust," but here's this: My name is Paul Pomeroy. I've been involved with the VCV community for a few years now (since March of 2018). You can find me in the VCV Rack Official User Group on Facebook (https://www.facebook.com/groups/122795801713906/user/1047045786) and on PatchStorage (https://patchstorage.com/author/paulpomeroy/). I was also "featured" in Omri Cohen's first "patch dissection" video back in August of 2019 (https://youtu.be/sSo8FHeoMwA). I've also posted VCV-realted videos on my YouTube channel (https://www.youtube.com/channel/UCCwydBkWZsaxO7tT5GFzYyA).
The code and patches listed on (and downloaded from) my GitHub page for That Rhythm Thing (https://github.com/PaulPomeroy/that-rhythm-thing) are written by me and are safe.