This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in MyBB.
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 application.
MyBB before 1.8.19
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected system.
Upgrade to the latest version, available from the web site.