What is the callback doing in lambda handler? It seems to be using sns variable and I intend to modify the variables.

exports.handler = function(event, context, callback) {
   console.log("AWS lambda and SNS trigger ");
   console.log(event);
   const sns = event.Records[0].Sns.Message;
   console.log(sns)
   callback(null, sns);   
};

I'm trying to modify the code to something like this and not sure what I should pass for callback. I also want to trigger another lambda and pass these new variables to it.

exports.handler = function(event, context, callback) {
   console.log("AWS lambda and SNS trigger ");
   console.log(event);
   const sns_NameSpace = event.Records[0].Sns.Message.Trigger.Namespace;
   const sns_ApiId = event.Records[0].Sns.Message.Trigger.Dimensions.value;
   const sns_MetricName = event.Records[0].Sns.Message.Trigger.MetricName;
   console.log(sns_ApiId + '_' + sns_MetricName)
   console.log(sns_NameSpace)   
   
   callback(null, sns); // Wondering what I should pass for the callback.
   
};