A ham is an object, person, word, or phrase in media that shows up once and becomes
important later
down the line.
The term comes from the 2019
Invader Zim movie, Enter the Florpus, where a literal ham was introduced into the story and later on was used in the final battle.
"Now pay close
attention to that ham, cuz it's not
the last time you're gonna see that ham in
the story." ... "See? There's that ham again."
"God, I had no idea the cat was gonna be a ham. It makes so much sense now."