A call stack is a data structure used by computer programs to keep track of active subroutines or function calls. It records the sequence of function calls that have been made up to the current point of execution, allowing the program to return control to the correct location after a function finishes executing. In cybersecurity, understanding the call stack is crucial for analyzing vulnerabilities such as stack overflows, which can lead to unauthorized code execution. Attackers may exploit flaws in the call stack to manipulate program flow, making call stack integrity a key focus in secure software development.