The Tender Validity Period refers to the duration of time for which a tender (or bid) submitted by a tenderer remains binding upon the tenderer.

What is Tender Validity Period?
Price offer submitted by a bidder cannot be valid for indefinite period of time because of price fluctuation and other considerations. Here comes the concept of Tender Validity Period which simply means that the offered price shall remain valid for a certain period of time which is popularly known as Tender Validity Period. This is a safeguard for the bidder who submit a tender against uncertain market volatility. A Procuring Entity (PE) cannot force a tenderer to extend Tender Validity Period or to accept a Notification of Award (NoA) offer after expiration of Tender Validity Period. So a Procuring Entity must issued a NoA within Tender Validity Period.
During this period, the tenderer is bound to the terms and conditions of their offer and cannot withdraw or alter it without risking forfeiture of the tender security (even other penalties) mentioned in the bidding documents. .

Points to be noted regarding Tender Validity Period:

Tender validity period will be normally between sixty (60) and one hundred twenty (120) days. But shorter or longer periods may be authorised by the Head of the Procuring Entity or an officer authorised by him. [Rule 19(2)]
A Procuring Entity has to complete evaluation of Tenders and issue NOA within the tender validity period. [Rule 20(1)]
Extension of Tender Validity: On justified ground Validity Period may be extended maximum two times by the PE subject to the consent of the bidders and approval from the HOPE (for 1st time) or higher authority (for 2nd time). [Rule 21(1)]
If a tenderer withdraws his Tender after the opening of Tenders but within the validity of the Tender security shall be forfeited. [Rule 25(21)]