This indicates an attack attempt against an UDP Amplification flaw on the Memcached protocol.
The vulnerability is due to an error in the vulnerable application when handling a series of maliciously crafted requests. An attacker can exploit this to cause a denial of service condition on the affected machine via maliciously crafted requests. The signature detects for 50 suspicious requests within 1 second.
Setting this signature to "Quarantine" is not definitive to who the attacker is as the both the client and the server are victims for this issue.
Memcached prior to 1.5.6
Denial of Service: Remote attackers can crash vulnerable systems
Apply the most recent upgrade or patch from the vendor: