Kotlin is new programming language that is challenging Java for building mobile apps on Android.
While there are many languages you can use to build Android apps, Google has chosen to commit to including support for Kotlin within its Android tools and IDE, alongside Java which is outdated (22 years old) and needs replacing.
Introductory refs:
- Kotlin (Wikipedia)
- Why Kotlin Might Replace Java (one of many comparisons if you search. This one is short and high level).
- Quick comparison of Kotlin v Java Language Features