It’s a calculator. Obviously. But what makes it better than what already exists? There’s a few things. Abacus is:
The initial project was proposed for the Northwest Advanced Programming Workshop. You can read the project proposal on the main GitHub page, although the idea has changed quite a bit, mostly in shifting from “fast” to “precise”.
Java and Kotlin. Java provides a good layer of abstraction and a great standard library, while Kotlin allows for the reduction of boilerplate code and null safety. Using JVM-based languages also allows Abacus to expose its entire API to plugins, and load them at runtime.