In Java, comments are used to add explanatory notes or documentation to the source code. There are two types of comments: single-line comments and multi-line comments.

Single-line comments start with two forward slashes (//) and continue until the end of the line. For example:

// This is a single-line comment
int x = 10; // This is also a single-line comment

Multi-line comments start with a forward slash and an asterisk (/) and end with an asterisk and a forward slash (/). Anything between these two markers is treated as a comment. For example:

/*
This is a multi-line comment.
It can span multiple lines.
*/
int y = 20;

Comments are not executed by the Java compiler and are not included in the compiled code. They are used to provide information to the reader about the code and are often used to document the purpose of a particular piece of code or to explain how it works. It is good programming practice to include comments in your code to make it easier to understand and maintain.