For ... Next repeats a block a set number of times with a counter, the standard loop for rows and columns.

Code

For i = 2 To lastRow
  Cells(i, 3).Value = Cells(i, 2).Value * 2
Next i

• Loop over rows

  For i = 2 To lastRow Cells(i, 3).Value = Cells(i, 2).Value * 2 Next i → Doubles column B into C for every data row

  Row loop.

• Delete rows

  For i = lastRow To 2 Step -1 If Cells(i, 1).Value = "" Then Rows(i).Delete Next i → Deletes blank rows safely from the bottom up

  Delete backwards so indexes stay valid.

• Step

  For i = 1 To 10 Step 2 → i = 1, 3, 5, 7, 9

  Step sizes.

• Find lastRow once before the loop, not inside it.
• Use Long for the counter.
• Exit For leaves early.

• Deleting rows in a forward loop skips rows; loop backwards.
• Changing the counter inside the loop is allowed but confusing.