.kt — normal source files, .kts — script files
The .kts files are those that run as script files, without a need for separate compilation. it is run using the following command:
kotlinc -script <filename>.kts
You don't need main function in a .kts file, It will be executed line by line just like a bash/python script.
.Kt and .Kts
As Explained in kolinConf18