Speculative execution is a performance optimization technique used in computer processors. It allows the CPU to predict which instructions might be needed next and execute them ahead of time, even before the actual need is confirmed. This can help improve the overall speed and efficiency of programs by reducing wait times. However, speculative execution can also lead to security vulnerabilities, such as those revealed in the Spectre and Meltdown exploits. These vulnerabilities arise when sensitive data is accessed during the speculative execution process, potentially allowing unauthorized access to information that should remain secure.