A KT file contains source code written in Kotlin, a statically typed programming language developed by JetBrains. Kotlin supports both functional and object-oriented styles and is most often used to supplement or replace Java when developing business and end user applications.
JetBrains developed Kotlin in order to alleviate some of the common issues that arise in Java development. For example, while Java is considered a verbose language, Kotlin is more concise and can reduce the number of lines of code by 40%. Kotlin is also is statically typed so that applications are less likely to produce Null Pointer Exceptions (NPEs) due to non-nullable type support.
Kotlin is 100% compatible with Java and is supported by major Java IDEs, such as Intellij IDEA, Android Studio, NetBeans, and Eclipse.