Complex sentence: It refers to protection of offspring (tending) and seeking out the social group for mutual defense (befriending) that was theorized as having evolved as the typical female response to stress, just as the primary male response was Fight-or-flight.
Split into simpler sentences: It refers to protection of offspring (tending) and seeking out the social group for mutual defense (befriending). Tend and befriend was theorized as having evolved as the typical female response to stress, just as the primary male response was Fight-or-flight.