Hello Narayan,
below is the definition from the PMBOK guide. think of it as a clarification session so that sellers can understand what is required and they can submit their proposals to match the requirements. so it has to done before the submittal of proposals.
"12.2.2.1 Bidder Conferences
Bidder conferences (sometimes called contractor conferences, vendor conferences, and pre-bid conferences)
are meetings between the buyer and all prospective sellers prior to submittal of a bid or proposal. They are used
to ensure that all prospective sellers have a clear and common understanding of the procurement requirements),
and that no bidders receive preferential treatment."