This indicates an attack attempt to exploit a remote Command Execution vulnerability in Apache Struts 2.
The vulnerability is due to insufficient sanitizing of parameters in the application. A remote attacker may be able to exploit this to execute arbitrary command within the context of the application.
Struts 2.0.0 - Struts 2.3.15
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest version available from the website.