Hi Deepak,
A question mark ( ? ) in Scheduled Apex in Salesforce is allowed in the day-of-month and day-of-week fields. It is used to specify “no specific value,” which is useful when you need to specify something in one of these two fields, but not in the other.
If you want a trigger to fire on a particular day of the month (for example, the 10th), but you don’t care what day of the week that is, enter 10 in the day-of-month field, and ? in the day-of-week field.