Meeting buffers calculate how much time ACE should wait before any person can book another meeting. For example, if you have a meeting that ends at 10 AM, and you want to have a 15-minute break before the next meeting can be booked (at 10:15 AM) - then you will have to set a post-meeting buffer of 15 minutes.
You can set them before and after a meeting with different values. For example, you can set a 5-minute buffer before a meeting, and a 10-minute buffer after a meeting.