Validity is the duration till when the order remains valid in the exchange. For example, a limit order placed in the morning remains valid in the exchange for a match until the market closes. 

Depending on the order type, you can choose the validity of your order for the following time periods:

IOC (Immediate or Cancel):  The orders are executed immediately. If the order is not executed immediately, it gets cancelled.
Day: The order is valid till the market closes i.e., at 3:30 pm on the day of order placement.