This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Apache Struts 2.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Struts 2.3.20 - Struts Struts 2.3.28 (except 188.8.131.52 and 184.108.40.206)
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's advisory for updates: