From AN 8.39 ( http://www.accesstoinsight.org/tipitaka ... .than.html
There is the case where a disciple of the noble ones, abandoning the taking of life, abstains from taking life. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. In giving freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings, he gains a share in limitless freedom from danger, freedom from animosity, and freedom from oppression. This is the first gift, the first great gift — original, long-standing, traditional, ancient, unadulterated, unadulterated from the beginning — that is not open to suspicion, will never be open to suspicion, and is unfaulted by knowledgeable contemplatives & brahmans. And this is the fourth reward of merit
So if someone abstains from killing other people, s/he basically offers a gift, the gift of freedom from danger, animosity, etc. to other people... As opposed to someone who kills, thus offering a curse which robs other people the freedom from danger, animosity, etc..
The "gains a share" part could mean both the immediate wholesome result one gets in the present life and the favorable rebirths in the future due to the wholesome kamma s/he's doing..