Please explain the pieces of this code

Tried my best to visualize what it does.

Sub transe()

    Dim a, z, i As Long, j As Long, Y, n&, x, k, jj&
    Const delim As String = ","
    
    With Sheets("Sheet1")
        a = .Range("A2").CurrentRegion                  'click on cell A2, and press Ctrl + A.
                                                        'this selects the "current region" and sets the
                                                        'values of this range as an array into variable 'a'
    End With
    
    With CreateObject("Scripting.Dictionary")           'go search it up on this
        .comparemode = 1                                'compares text so "AAA" is equals to "aaa"
                                                        'i.e. case-insensitive
                                                        
        ReDim Y(1 To UBound(a, 1), 1 To 2)              'dim variable Y with the number of rows you have, and with 2 columns
        For i = 2 To UBound(a)                          'loop through the rows
            If Not .Exists(a(i, 1)) Then                'check if a unique key entry exists in the dictionary object
                                                        'let's call this KEY1
                                        'does not exist
                ReDim z(1 To UBound(a, 2))              'dim variable z to the maximum number of columns you have
                For j = 1 To UBound(a, 2)               'loop through the columns
                    z(j) = a(i, j)                      'put into array z each data you have (through the columns)
                Next
                .Item(a(i, 1)) = Join(z, delim)         'join up the array z with the delimiter as defined above ","
                                                        'e.g. abc,def,ghi,jkl,,,,  is now stored in KEY1
            Else
                                        'exists
                ReDim z(1 To UBound(a, 2))              'same as above
                For j = 2 To UBound(a, 2)               'same as above
                    z(j) = a(i, j)                      'same as above
                Next
                .Item(a(i, 1)) = .Item(a(i, 1)) & "," & Join(z, delim)
                                                        'since a unique key exists, it now adds what was joined into existing data
                                                        'e.g. abc,def,ghi,jkl,,,,opq,rst,uvw,xyz,,,,  is now stored in KEY1
            End If
        Next
        z = .items                                      'variable z now holds all items stored (with "," delimiter)
        For i = 0 To UBound(z)                          'loop through the items
            x = Split(z(i), delim)                      'split each item z holds
                                                        'e.g. x now holds
                                                        '   "abc"
                                                        '   "def"
                                                        '   "ghi"
                                                        '   "jkl"
                                                        '   ..etc.
            n = n + 1                                   'row counter
            jj = 0                                      'reset column counter (on each row looped)
            For k = 0 To UBound(x)                      'loop through x. i.e. "abc" first, then "def", then "ghi", etc.
                If x(k) <> vbNullString Then            'check that it's not a blank
                    If jj >= UBound(Y, 2) Then ReDim Preserve Y(1 To UBound(a, 1), 1 To jj + 1)
                                                        'jj is the column counter, if there is more columns than the array Y
                                                        'currently holds, "upsize" array Y to fit
                    jj = jj + 1
                    Y(n, jj) = x(k)                     'we know that variable Y has the same number of rows you have
                                                        'and just enough columns to fit the largest row with most columns
                                                        'this now puts data into Y
                                                        'e.g.
                                                        'KEY1   "abc"   "def"   "ghi"   "jkl"   "opq"   "rst"   "uvw"   "xyz"
                End If
            Next
        Next
        
        With Sheets("Sheet2")
            .UsedRange.Offset(1).ClearContents          'go to sheet2, select your "used range" and move 1 row down. i.e. cell A2 onwards
                                                        'and clear whatever the cells contains
            .Range("A2").Resize(n, UBound(Y, 2)) = Y    'from A2, select the size (rows & columns) that you have in Y and sets the values in
            .Columns.AutoFit
        End With
    End With
    
End Sub

HTH