This indicates an attack attempt to exploit a remote Command Execution vulnerability in WordPress Plainview Activity Monitor plugin.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. An authenticate remote attacker may be able to exploit this to execute arbitrary OS commands within the context of the application, via a crafted HTTP request.
WordPress Plainview Activity Monitor plugin before 20180826
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.