The Valid Till option specifies the duration for which the order will be pending.
Day (the default) means the order will be pending till the end of day if not executed. In BSE, I think the word SES (session) is used and means the same thing.
IOC (Immediate or Cancel) specifies that if the order is not filled in immediately it is cancelled. The fill may be full or partial or not at all. In Level 2/Best 5, you sometimes see large order that vanish in the next tick; they are such orders.
Dis Qty (Displayed Quantity) is the quantity you would like to be shown in Level 2/Best 5. For example, very frequently you see orders with a quantity of 1; traders are hiding the actual order quantity.