This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in WordPress plugin Strong Testimonials.
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 script code within the context of the target user's browser.
Strong Testimonials Plugin 2.40.0 and before
System Compromise : Remote attackers can execute arbitrary script code within the context of the target user's browser
Upgrade to Strong Testimonials Plugin 2.40.1