The lock-up exists so the small float created by an IPO is not immediately swamped. When it expires, the tradeable supply can multiply several times over in a single session, which is why the expiry date is calendared by traders from the day of the debut.
Modern lock-ups often include early-release triggers: a portion unlocks after the second earnings report, or once the stock trades a set percentage above the offer price for a number of days. Underwriters can also waive the lock-up, and frequently do around a follow-on-offering.
Example: 20M shares float after the IPO and 180M are locked. At expiry the potential float rises tenfold. If even 8% of locked shares are sold, 14.4M shares hit a market used to absorbing 2M a day.
Related: ipo, float, restricted-stock, form-144, secondary-offering