@Explain(displayName = "Statistics", explainLevels = { Level.USER })
  public String toUserLevelExplainString() {
    StringBuilder sb = new StringBuilder();
    if (runtimeStats) {
      sb.append("runtime: ");
    }
    sb.append("rows=");
    sb.append(numRows);
    if (runTimeNumRows >= 0) {
      sb.append("/" + runTimeNumRows);
    }
    sb.append(" width=");
    // just to be safe about numRows
    if (numRows != 0) {
      sb.append(dataSize / numRows);
    } else {
      sb.append("-1");
    }
    return sb.toString();
  }
Statistics. Describes the output of an operator in terms of size, rows, etc
based on estimates.
ql/src/java/org/apache/hadoop/hive/ql/plan/Statistics.java