Comments are used in C (and other programming languages) to add notes, explanations, or documentation to the source code. Comments are ignored by the compiler and do not affect the behavior of the program. They are used to make the code more readable and understandable for the programmer and other developers who may work on the code in the future.

There are two types of comments in C:

1. Single-line comments: These comments start with the double slash (//) and continue until the end of the line. Everything on the same line after the double slash is considered a comment and ignored by the compiler.

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// This is a single-line comment

1. Multi-line comments: These comments start with a forward slash and an asterisk (/) and end with an asterisk and a forward slash (/). Everything between the start and end markers is considered a comment and ignored by the compiler.

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/* This is a multi-line comment */

It's important to note that comments should be used sparingly and only to add information that is not already clear from the code itself. They should be used to explain the purpose of the code, describe how it works, or provide other relevant information that is not obvious from the code. Additionally, it's good practice to keep comments up to date and accurate, as outdated or incorrect comments can be more confusing than helpful.