Kin selection refers to the idea that animals can be more likely to engage in self-sacrificial behaviour if the individual benefiting from it is closely related. According to this hypothesis, the individual lowers it’s own individual reproductive fitness, but at the same time increases the fitness of the genes which it and its kin members share.

Source:

Smith, J. M. (1964) Group selection and kin selection. Nature 201,1145–1147.