In a hierarchical network design, you usually have three levels (you dont have to have all three) of networking gear.
Access is for connecting to the network. Like the switch your PC connects to. This layer is mostly most secured.
Distribution is the layer that connects many access devices together. If the network is huge you can see multiple distribution layers. If network is small, you may not see it at all.
Core is where you have all your distribution connect to. Usually you have little or not security at this layer. it is meant to switch packets as fast as it can.
HTH.
