In C, the volatile keyword is a qualifier that is applied to a variable when it is declared. It tells the compiler that the value of the variable may change at any time without any action being taken by the code the compiler finds nearby. The implications of this are quite serious, as it prevents the compiler from applying any optimizations on objects that can change in ways that cannot be determined by the compiler.

The volatile keyword is used to indicate to the compiler that a variables value may change unexpectedly, which is often the case when a variable is memory-mapped to a peripheral device or shared between multiple threads). When a variable is declared as volatile, the compiler generates code to take the value of the given variable each time it is accessed, rather than caching the value in a register or optimizing it in any other way).

In conclusion, the volatile keyword is used to prevent the compiler from making certain assumptions about a variable and to ensure that the value of the variable is read and written to memory, rather than being optimized). It is an essential tool for certain programming scenarios, such as multi-threaded programming, embedded programming, and when working with hardware devices).