Cloudwatch alarm: "cb9978ge CPUUtilization"

I'm new with AWS and I'm having a very stupid question. I'm getting AWS Notifications emails a few times a day with the following content:

You are receiving this email because your Amazon CloudWatch Alarm "cb9978ge CPUUtilization" in the US-West-2 region has entered the ALARM state, because "Threshold Crossed: 1 datapoint (99.666) was greater than or equal to the threshold (80.0)." at "Tuesday 03 December, 2013 04:33:06 UTC".

Alarm Details:
- Name:                       cb9978ge CPUUtilization
- Description:                cb9978ge CPUUtilization
- State Change:               OK -> ALARM
- Reason for State Change:    Threshold Crossed: 1 datapoint (99.666) was greater than or equal to the threshold (80.0).
- Timestamp:                  Tuesday 03 December, 2013 04:33:06 UTC
- AWS Account:                468642797431

Threshold:
- The alarm is in the ALARM state when the metric is GreaterThanOrEqualToThreshold 80.0 for 300 seconds. 

Monitored Metric:
- MetricNamespace:            AWS/EC2
- MetricName:                 CPUUtilization
- Dimensions:                 [InstanceId = i-cb9978ge]
- Period:                     300 seconds
- Statistic:                  Average
- Unit:                       not specified

State Change Actions:
- OK: 
- ALARM: [arn:aws:sns:us-west-2:468642797431:cb9978ge_CPUUtilization_80percent_for5mins_ormore]
- INSUFFICIENT_DATA: 

Does anyone know what it means?

Thanks a lot

the alarm that you created is cb9978ge CPUUtilization.

This alarm will monitor instance i-cb9978ge for CPUUtilization. If Average CPUUtilization of this instance goes beyond 80% for a consecutive 300 seconds, then this Alarm gets triggered.

The trigger action for this alarm is to send you an email via SNS topic.