Single-line comments use a double slash (//):

// This is a single line comment

println("Hello, World!") // This is a single line comment too, after valid code

Block comments use a slash and asterisk to open the block (/*) and an asterisk and slash to close (*/):

/*
This is a multi-line comment,
Roses are red
... and I forgot the rest

*/

println(/*block comments can be inside valid code*/ "Hello, World!")