Walter White, cancer patient and kingpin in utero, has a drug dealer named Crazy Eight trussed up in his basement. Walter is new to Albuquerque's criminal underworld, and he figures that prisoners need to eat like anyone else, right? So he makes Crazy Eight a turkey sandwich and carries it down the stairs on a heavy yellow ceramic plate.
The sandwich never makes it to Crazy Eight, though, because Walter faints -- his health is deteriorating quickly. When he does, the plate smashes, and the sandwich is ruined. Walter comes to and has a groggy conversation with Crazy Eight that feels human. He dusts himself off, scoops up the shards of ceramic, and tells his prisoner that he'll go make him another sandwich. Upstairs, Walter tosses the plate in the trash, turns towards the fridge... and has a thought. He reaches back into the trash, grabs the remnants of the plate, and tries to piece it together. But there's a piece missing, a long and jagged one.
That missing piece of yellow ceramic is what a semiotician would call an "indexical sign", indicating Crazy Eight's intention to murder Walter. As another example, smoke is an indexical sign for fire. These are much more literal signs than the other options, iconic signs (the men's room has a little stick figure on the door) and the highly abstract symbolic signs (the word, *ceramic*, is understood to denote a particular nonmetallic, brittle material).
An indexical sign can give you great moments of dread and suspense. A cup of water trembles, indicating the approach of (say) a T-rex. A man comes home, looking for his wife, and finding a blood splatter. A British soldier on a beach hears the rising whine of a Messerschmitt.
But I want to go back to the yellow plate, because it's a slightly more subtle piece of signing. Those others example I listed are direct. Blood on carpet is a Bad Thing. The sound of an enemy fighter plane is a Bad Thing. A trembling cup of water is a little more sophisticated, but essentially: Big Thing is Bad Thing.
But that yellow plate was a peace offering, with a gift of food. (Yellow is a nice friendly color, too.) So there is an emotional beat to be had, along with the dread, as Walter assembles the plate. That symbol of friendship has been corrupted, and now signifies death.