Hystrix is not actively mainatined anymore and some of its goals can now be handled with service meshs.
Hystrix is a very powerful library for handling failures, fallbacks and latency management within complex distributed environments. Netflix developed it and after years of experience, they are using it in almost each of their microservices. It evolved to a great library for handling resilience in complex architectures and covers solutions for the most common resilience patterns like:
Beside from that purposes Hystrix also offers some helpful features like parallel and asynchronous execution, In-Request-Caching and other useful features for working with distributed systems.
Another useful component that you are able to use with Hystrix is his dashboard that give you the ability of real time monitoring of external dependencies and how they behave. Alerting is also able via the dashboard.