Checksec is a bash script to check the properties of executables (like PIE, RELRO, PaX, Canaries, ASLR, Fortify Source). It has been originally written by Tobias Klein and the original source is available here: http://www.trapkit.de/tools/checksec.html
Modern Linux distributions offer some mitigation techniques to make it harder to exploit software vulnerabilities reliably. Mitigations such as RELRO, NoExecute (NX), Stack Canaries, Address Space Layout Randomization (ASLR) and Position Independent Executables (PIE) have made reliably exploiting any vulnerabilities that do exist far more challenging. The checksec script is designed to test what standard Linux OS and PaX (http://pax.grsecurity.net/) security features are being used.