what is a line driver, i was told that i need one

 

it boosts the output voltage of your deck. usually to 8vrms

 

does it make a big difference?

 

Pretty much just as WD21 stated.
If your HU has outputs of say 2v and your amplifier can accept up to 5v input then you can use the line driver to boost the signal to 5v and therefor set the gains on the amplifier lower and lower the noise floor. If you don't have a problem with noise (hiss for one example) then adding a line driver won't necessarily make your system sound any better.

Why have you been told that you require a line driver?

 

who makes a good liner driver & what do they sell for?