When a visitor comes to your web pages and clicks on an affiliate ad, a "cookie" is stored in that visitors web browser cache. If the visitor goes back to the merchants web site the next day and buys something, that cookie is read and you are credited for the sale.
Cookie "stuffing" is any trick which puts the cookie into the visitors web browser cache without the visitor clicking on an affiliate link.
There are quite a few ways to do this, but I think that it's best for me to help you stay out of trouble by not telling you. Cookie stuffing can get you banned from affiliate programs and may get you sued or even charged in criminal court.