A post reach is how many people see what you post while post engagement are those who engaged with the post in some way such as a comment, like or share. Knowing the difference is important to what your goal is in creating the post. If you want to let people know you exist and reach as many people as possible, you would want to be concerned with the post reach. If you are more interested in engaging with your customers and building customer loyalty, you would focus on the post engagement and how many are responding to what you posted.
Analyzing my own business page, Shanti Kitti Yoga, I can see that in the last 2 weeks, the highest post reaches came from pictures of sunsets, a poem by Pablo Neruda, a cute bunny video and a video explaining why your cat would feel like biting you. The most engagement came from the bunny video and the cat video. So I probably want to post more interesting videos to engage my customers.
If you love bunnies, click below to view the short video.