Please help me with this code ...i dont understand

The task to do :

Col A Col D

100 4
101 2
102 4
105 2
100 1
101 2
102 3
1055 1

After my code is run, I need for my spreadsheet to look like this

Col A Col D
100 5
101 4
102 7
105 2

And there was one guy post his solution code below :

Sub Test()

    Dim Sh As Worksheet
    Dim LastRow As Long
    Dim Rng As Range
    Set Sh = Worksheets(1)
    Sh.Columns(5).Insert

    LastRow = Sh.Range("A65536").End(xlUp).Row
    With Sh.Range("A1:A" & LastRow).Offset(0, 4)
        .FormulaR1C1 = "=IF(COUNTIF(R1C[-4]:RC[-4],RC[-4])>1,"""",SUMIF(R1C[-4]:R[" &      LastRow & "]C[-4],RC[-4],R1C[-1]:R[" & LastRow & "]C[-1]))"
        .Value = .Value
    End With

    Sh.Columns(4).Delete
    Sh.Rows(1).Insert
    Set Rng = Sh.Range("D1:D" & LastRow + 1)

    With Rng
        .AutoFilter Field:=1, Criteria1:="="
        .SpecialCells(xlCellTypeVisible).EntireRow.Delete
    End With

End Sub

Now , based on this code, i have too much confusions:

1: why did he insert column 5?
2, how to understand the COUNTIF systax ?? What is [-1] etc
3,what did he mean for deleting column5, and insert Row (1)??
4,why did he use autofilter?

i am sorry for my confusions, this code is realy awsome, just want to know how does it work,,, because i am a noob now....

Many thanks for any help !!!!!!!!!!!!

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