Nuclear stress rule

Hey, linguists, could any of you explain the Nuclear Stress Rule to me? I'm having a lot of trouble understanding it because I'm really not familiar with the Chomskyan side of the field. (I'm all sociolinguistics and discourse analysis here, so I would super appreciate any help.)

Hahaha, I have totally been abusing my fandom journal for linguistics lately, but I have only one linguist friend on my regular journal and I already asked her. She doesn't get it either.