A pop is money that went to allocated investors rather than to the company. Banks argue a modest pop rewards the buyers who took the risk and builds aftermarket support; issuers increasingly see it as leaving cash on the table, which is part of the case for a direct-listing.
For traders the pop creates a first print far above the reference the deal was built on, and no supply from index funds or analyst-coverage yet. The first weeks are often the least informed pricing a stock will ever see.
Example: priced at $28, opens at $44.80, closes at $41. The pop is 60% at the open. On a 20M share deal, the $16.80 open gap is $336M of value transferred from the issuer to allocated buyers.
Related: ipo, ipo-allocation, direct-listing, syndicate-stabilization, ipo-price-range