hi to make a driver key and what does really a driver do.and how important is it ? can we make an animation without that ?

Drivers can not be keyframed. Instead, they present their value to the system based upon a connection with other values in the system. You can also specify a python function to calculate some value to return for the field.

To create a driver simply right click on a value and choose Add Single driver. The field will turn purple. Now for the tricky part. You have to open up the Graph Editor and switch it to Driver mode (it defaults to F-Curve at bottom). Then your driver will show up as a channel. Click the channel and press the N-KEY to make sure the numeric panel opens on the right. Scroll to the bottom of the numeric panel and you will find the setup area for routing data from other objects in the scene to your drivers value.

Yes, you can make animations without using drivers at all. Typically they are used in RIGs to emulate the logic of a rig. For instance I might connect a driver to a lever that relays the x location to the y rotation of a wheel.

Proximal effects can be setup using drivers as well. Say you want a light to get brighter if a sphere is close to it.