Question

explain the structure of the lateral line system in fish

Answer 1

It is a sensory organ that allows aquatic vertebrates to sense movement by way of tiny hair cells.
is that enough detail or do you need more?

Answer 2

i need to know about the structure and function

Answer 3

http://www.britannica.com/EBchecked/topic/331503/lateral-line-system

There's a good picture there along with some description which might help you figure it out.
Basically there is a system of receptors under the skin/mucus layer which are attached to nerve endings that are triggered when the fish moves through the water, or if something moves the water around the fish.