Increases the rate of fire for crossbows to two attack per round. Otherwise you can only shoot one crossbow bolt per round.
Note: Taking Rapid Reload also opens up the use of the feat Rapid Shot with all crossbows. Rapid Shot can add another attack per round, bringing the total number of bolts fired per round to three. Crossbows are arguably of diminished use once a character has more than three attacks per round. As a result, a crossbow is usually a poor choice of weapon in combat, with Rapid Reload being a wasted feat.
However, it should be noted that the two / three attacks performed are made at the highest two / three attack bonuses. This means that you are potentially saving ammunition fighting high-AC targets as the fourth to sixth attacks have lower chances to hit. A bow is still a better choice when fighting low-AC targets and it usually pays more to take martial weapons feat to gain access to bows and the majority of other weapons (in cases of non-multiclassing caster characters that don't have access to bows on the start).