This indicates an attack attempt to exploit a Code Execution vulnerability in the Wordpress MailChimp Subscribe Form Plugin.
The vulnerability is due to an insufficient checking over user supplied data when the vulnerable application handles malicious requests. A remote attacker may be able exploit this to execute arbitrary codes in the affected machine via crafted requests.
WordPress Project MailChimp Subscribe Forms Plugin 1.1
System Compromise: Remote attackers can execute arbitrary code in the context of the affected site
Apply the most recent upgrade or patch from the vendor