The explain generally displays the plan which would be prepared by Optimizer.We can say that(though not accurate) plan would be the steps followed by Teradata to solve a query.Confidence levels indicate how well the optimizer is aware about the demographic data for a particular step.
High confidence: Means the optimizer know about the no of rows that would be returned as a result of that step. Examples would be when PI statistics exist when the columnor range stats exist or no join involved.
Low confidence: Some stats available. Join and stats available on both sides of the join.