The phrase "covert advertising" refers to advertising that is hidden in other media, such as an actor in a movie drinking a Coca-Cola. It is referred to as "covert" because it is not direct advertising, but subliminally viewers often notice the product.
Many films and other media use covert advertising because advertisers pay for the inclusion of their product within a film. Examples of some covert advertising include the use of Cadillac cars in the film "The Matrix Reloaded," and Converse shoes in the film "I, Robot." Viewers may not always directly notice the product being advertised, but the item is often covertly placed so that the consumer will indirectly notice it.