Ham is a meat product made from pigs, which appears several times in the Invader Zim series.
In "Tak: The Hideous New Girl", the second half of the episode opens with Zim randomly fighting a Ham Demon, which he ends by tossing it into the distance. No explanation is ever given for how this came to be.
In "The Girl Who Cried Gnome", when Dib is forced to buy Girly Ranger cookies, he pays them with the first thing he pulls out of his pocket, which happens to be a ham, to the anger of the Rangers. Dib is confused as to why he had ham in his pocket.
In Issue 15, when Ms. Bitters is late coming to class, the Skool provides a ham as a substitute teacher (due to not having the money for an actual substitute). Later, when Zim makes the random accusation that the ham is actually Ms. Bitters in disguise, a frustrated Dib smashes the ham (aided by a "ham-mangler" that Zim provides him). Ms. Bitters then appears and gives Dib detention for failing to "respect the ham".