An include guard is the #ifndef X_H / #define X_H / ... / #endif wrapper around a header's contents. Because #include literally pastes a header's text into each file that includes it, and headers often include other headers, the same header can be textually included multiple times in one translation unit, causing "redefinition" errors for its typedefs, structs, and macros. The guard makes the first inclusion define the guard macro and process the body; every subsequent inclusion sees the macro already defined and skips the entire file. #pragma once is a widely supported non-standard alternative that's simpler and avoids guard-name collisions, but the #ifndef form is universally portable. Either way, every header should have one.
C Programming · Interview question
What is an include guard and what problem does it solve?
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From the lesson
The Preprocessor
Text substitution before the compiler ever runs: object and function macros, header guards, conditional compilation, and why a naive MAX macro evaluates its argument twice.