What do "reduced antipredator defense" and "reduced antipredator vigilance" mean?
When groups of prey use certain mechanisms to defend against predators, those mechanisms are known as antipredator defenses. For example, groups of tuna and starlings flock together to confuse predators. A reduction in this defense would be 'reduced antipredator defense'.

Groups of prey also exhibit shared antipredator vigilance, as in certain members of a group will look out for predators to protect the group. This is seen most predominantly in meerkats. A reduction is said vigilance would be a 'reduced antipredator vigilance'.

With reference to the Allee effect, which is what you appear to be reading about, this reduced antipredator defense and vigilance can significantly effect the population and the population growth. 

Some related links:


On the ecological mechanism of the Allee effect.
Vigilance, predation risk, and the Allee effect in desert Bighorn sheep, a recent research paper.