what does this code mean can any one tel me plz

It's a Conditional operator: '?:' which is effectively an "inline-if" statement.

A = B ? C : D;

If B is true, then C is assigned to A, otherwise D is.
So your code could be written as:

var startDateParameter;
if (startDate.HasValue)
   {
   startDateParameter = new ObjectParameter("StartDate", startDate);
   }
else
   {
   startDateParameter = new ObjectParameter("StartDate", typeof(System.DateTime));
   }

which should be a bit more obvious.
Indenting the original mighthave helped clarity:

var startDateParameter = startDate.HasValue ? new ObjectParameter("StartDate", startDate) 
                                            : new ObjectParameter("StartDate", typeof(System.DateTime));

Or

var startDateParameter = startDate.HasValue 
                         ? new ObjectParameter("StartDate", startDate) 
                         : new ObjectParameter("StartDate", typeof(System.DateTime));

Perhaps.