An event handler is just an event that is called when a user clicks on or otherwise actions a control, such as your DataGridView.
Clicking on the DataGridView, will trigger the On_Click event. The first line of code after the declaration gets the ROW number of the grid, which in turn is used to access the data from the cells. For example......
Private Sub DataGridView_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles DataGridView.Click
'Declare a couple of variables to hold the row number and contents of the third cell
Dim pRowNo as Integer
Dim pstrContents as String
'Get the ROW number
pRowNo = Val(DataGridView.SelectedCells(0).RowIndex.ToString())
'It's here where you test if Cell 0 is checked, then if so the remainder (below) is actioned.
'Store contents of cell three
pstrContents = DataGridView.Rows(pRowNo ).Cells(2).Value
'Insert contents into cell two
DataGridView.Rows(gintROWNO).Cells(1).Value = pstrContents
This should get you started, you will find some great examples using Google.
Remember, this will place the data in the cell but will not save ot to your data source.