cisco mc3810-v3 access concentrator what does this do?

I have a network here at work. We have our T1 coming in, and it goes from our main line into our T1/E1 port on this access concentrator. We have a new internet connection coming in that i need to setup, and im wondering about how this should be configured.

We have a cisco 1700 router this is connected to also, by the serial connection, big plug on the back.

I also need to reset the passwords on them, which I am doing today.

What does this unit actually do?? any help woudl be nice.