ReDim instruction is used to resize an array. It is useful to save memory in case you have an array too big for your purpose or for dinamic array management.

It ca be used in conjunction with Preserve parameter, in case you want the data already memorized not to be erased

Sub redm()
    Dim arr() As String
    ReDim arr(0)  

    arr(0) = "value1"

    ReDim Preserve arr(2) 'now: three element array

    arr(1) = "value2"
    arr(2) = "value3"

End Sub

In the example above the one element array is extended to a three element one.
The ReDim parameter includes also the 0 value: you have to consider then the number of elements based on n-1 formula.