So what does it mean for stress to be good for our brains, specifically regarding the hippocampus?

We looked at two mechanisms. Something important for the hippocampus and its function regarding memory is the generation of new neurons and adult neurogenesis. In most of the brain, there is a limited amount of neurons that you have that gradually died. However, there are a couple of areas with a generation of neurons through life and a pool of stem cells that create new neurons. These are important for specific memories, and they’re very important for your HPA axis (your stress axis). So they play a role in both regulating your stress and memory. As you age, you generate less neurons but you still generate them even up until around 80 or 90. Chronic stress depresses that, and decreases the generation of these new neurons. However, a moderate amount of stress actually pushes it up and helps generate more neurons that are then integrated into your circuitry and activated in your fear learning to buffer you when the next stressor comes along.