A Buffer is a container with 5 different compartments for items. These compartments correspond to inputs from 5 different sides and are filled individually. The last side, the output side, has access to all of the compartments.
By using different compartments for each of the input sides, the buffer provides an easy solution for when items of different types arrive at very different rates. Using a normal chest in this situation may risk filling up the chest with only items of one kind.