how does missing data work in cloudwatch alarm?

A CloudWatch alarm for API Gateway endpoint. Based on documentation, I have created template below and way I read it, is if there is 5xx error, which will be greater than '0' threshold, this should trigger. I'm not sure about the TreatMissingData attribute below, what data does this refer too? Also not sure about the EvaluationPeriods attribute as well? can someone explain on it?

loudAlarm:
  Type: AWS::CloudWatch::Alarm
  Properties:
    Namespace: AWS/ApiGateay
    MetricName: 5XXError
    Period: '60'
    EvaluationPeriods: '1'
    Threshold: 0
    Statistic: Sum
    ComparisonOperator: GreaterThanThreshold
    TreatMissingData: ?????????
    AlarmActions:
      ...

TreatMissingData is a parameter how to do when the data is missing.

Values are:

• breaching: missing data is treated as breaching threshold
• notBreaching: treated as within threshold
• ignore: missing data is ignored
• missing: ignored, if all of data missing in the period range, alarm INSUFFICIENT_DATA

if not specified, missing is used.

EvaluationPeriods is a parameter how many times the threshold has to be breached for alarm.

https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/AlarmThatSendsEmail.html