The intro video on the homepage does a good job at explaining it in 3 minutes.
The video is not upselling. The tool can do what the video promises.
Say your system is "well tested" for example with a combination of unit tests, integration tests, stress tests, failure injection tests (a la Jepsen), and more.
There are probably still hundreds of nasty little bugs hidden. Not even thousands of experiment hours with a Jepsen-like approach would surface them because they are very, very unlikely.
Antithesis will find these without breaking a sweat. It's designed to hunt for very unlikely (but possible) scenarios where your system misbehaves.
And here's the real kicker: once a bug is found, you can observe and step-through execution of your entire _distributed_ system. Similar to attaching a debugger to a single process but for an entire system composed of many clients and servers connected by a network.
It's language independent and doesn't require any modification to your system in order to use. It's pretty incredible. I would not believe this is possible if it hadn't seen it with my own eyes.
Presently, I am using it for work at Synadia (makers of NATS). NATS is like lego blocks to build all kinds of distributed systems in multiple languages, so it has a very large surface area. It's well tested, stable, and deployed successfully by many large and small projects and companies. Contemporary testing approaches can hardly find bugs. Antithesis can find very insidious edge cases where things break. And we proactively investigate and fix before any user/customer can be affected by these one-in-a-million nasty bugs, which would otherwise be very hard to find and resolve.