You just activated my trap card! is a warning phrase which indicates the recipient has dun goofed and is about to be owned. It is the memetic equivalent of prefacing an argumentative rebuttal with the phrase “well actually…” and implies an attack is imminent.
Other types of Yu-Gi-Oh card parodies are also used with this meme on occasion. They include symbolism to indicate relevant strengths and attributes, with stats most often maxed out. Fake CCG cards are not a derivative of this meme as they predate it by at least a decade.