please can you tell me in easy words what is CSU/DSU and what is it for..?

A DSU/CSU in the simplest terms is a media converter. Most typically it converts the channelized T-1 or E-1 signal from the telco network to a high-speed serial signal stream for connection to a router.

In more technical terms it's a Digital (or Data) Service Unit/Channel (or Customer) Service Unit that connects Data Terminal Equipment such as a router to a digital line such as a T-1, E-1 or T-3 line. In the US a DSU/CSU is a piece of CPE or Customer Provided (or Provisioned or Premises) Equipment. In many other countries it's CE or Carrier Equipment.

A DSU/CSU is a specific type of equipment. A Cisco WIC-1T WAN interface card or an Adtran TSU series interface are good examples.

Connects your digital network device (usually router) to a transfer median (T1, T3). It also keeps track of errors. It's like a little box that you see connected to a router and the lines running out of it go to a phone companies lines.

Channel Service Unit/ Data Service Unit