What does object[,] mean in c#?

So I came across some code of mine from an old VSTO project, and noted this little bit:

Excel.Worksheet sheet = Globals.ThisAddIn.Application.Worksheets["Unique Hits Per URL"];
        Dictionary<int, string> ids = new Dictionary<int, string>();
        object[,] cellRange = (object[,])sheet.get_Range("E:E").Cells.Value;
        for (int i = 1; i < cellRange.GetUpperBound(0); i++)
            if (cellRange[i, 1] != null)
                ids.Add(i, cellRange[i, 1].ToString());

What does specifying [,] on a datatype mean? Looking at the code it appears to function as a matrix, but honestly I thought c# matrices were handled with notation like object[ ][ ].