Turn signal swap to drl's

[NS633]

What will I need to fix this led as a drl with the switchback option for my turn signal?
http://www.ebay.com/itm/322309522173
They say that I need a load resistor.
I don't know how to and why I should install a load resistor?
Please explain me about this light swap.

 

[drdel]

Might be illegal in some markets