I believe the GP is talking about the trend of adverts for the exact product you just viewed/bought on the exact store following you around the Internet for a few days.
e.g. you bought a blue striped t-shirt from H&M and it is continually advertised to you by H&M for a few days.
e.g. you bought a blue striped t-shirt from H&M and it is continually advertised to you by H&M for a few days.