A Just-In-Time (JIT) compiler is a type of compiler that improves the execution speed of programs by converting code into machine language at runtime, rather than beforehand. This allows the program to run faster because the JIT compiler can optimize the code based on the current execution context and usage patterns. JIT compilers are commonly used in environments like the Java Virtual Machine (JVM) and the .NET Framework. By compiling code on-the-fly, they can take advantage of the specific hardware and system resources available, leading to better performance compared to traditional interpreters that execute code line by line.