Q:

Can somebody explain me what this VBA code means?

This is for an excel with macros. I am really lost here. I'd really appreciate any help.  
Sheets("5. Resume").Select

ActiveSheet.PivotTables("PivotTable1").PivotSelect "'Tarea IS'[All]", _
    xlLabelOnly, True
ActiveSheet.PivotTables("PivotTable1").PivotCache.Refresh

ActiveSheet.PivotTables("PivotTable2").PivotSelect "'Tarea IS'[All]", _
    xlLabelOnly, True
ActiveSheet.PivotTables("PivotTable2").PivotCache.Refresh

Sheets("6. Reg Err").Select
ActiveSheet.PivotTables("PivotTable3").PivotCache.Refresh 

A:

I'll comment your code for you if this helps:
'Select the sheet named "5. Resume"
Sheets("5. Resume").Select

'Select the pivot table named "PivotTable1" on the active sheet ("5. Resume")
ActiveSheet.PivotTables("PivotTable1").PivotSelect "'Tarea IS'[All]", _
    xlLabelOnly, True

'Refresh the data in "PivotTable1" on the selected sheet ("5. Resume") from the source
ActiveSheet.PivotTables("PivotTable1").PivotCache.Refresh

'Select the pivot table named "PivotTable2" on the active sheet ("5. Resume")
ActiveSheet.PivotTables("PivotTable2").PivotSelect "'Tarea IS'[All]", _
    xlLabelOnly, True

'Refresh the data in "PivotTable1" on the selected sheet ("5. Resume") from the source
ActiveSheet.PivotTables("PivotTable2").PivotCache.Refresh

'Select the sheet named "6. Reg Err"
Sheets("6. Reg Err").Select

'Refresh the data in "PivotTable3" on the selected sheet ("6. Reg Err") from the source
ActiveSheet.PivotTables("PivotTable3").PivotCache.Refresh 

The lines where you select the pivottable seem to be irrelevant as you are changing the selection shortly after without doing anything to the currently selected item (I am guessing this is recorded). You can remove the lines with .PivotSelect and it will not change the outcome.
ActiveSheet.PivotTables("PivotTable1").PivotSelect "'Tarea IS'[All]", _
        xlLabelOnly, True

In fact, the following code achieves the same in a much shorter fashion:
With Sheets("5. Resume")
    .PivotTables("PivotTable1").PivotCache.Refresh
    .PivotTables("PivotTable2").PivotCache.Refresh
End With
Sheets("6. Reg Err").PivotTables("PivotTable3").PivotCache.Refresh

