This indicates an attack attempt against a Denial of Service vulnerability in Apache Tomcat.
The vulnerability is due to an error when vulnerable software handles a crafted HTTP/2 request. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Apache Tomcat 9.0.0.M1 to 9.0.0.M18
Apache Tomcat 8.5.0 to 8.5.12
Apache Tomcat 8.0.x and earlier are not affected
Denial of Service: Remote attackers can disrupt services on vulnerable systems.