Some atoms don't like being the way they are (mood tbh), and we call them unstable. Some unstable atoms spit out something called a positron in order to become the atom they want to be.

A positron is an anti-electron, and when it collides with an electron (easy since those are everywhere) they annihilate each other, turning into a bunch of energy in the process, in the form of gamma radiation. The gamma radiation from annihilation is special because it always comes out as two rays going in the opposite direction from each other.

That means, if you can detect when those two rays hit a ring that encircles the point of annihilation, you can use math to figure out where that point is in the ring. That's because you know the speed of the rays and the difference in the times that both rays struck the ring.

The aforementioned ring is a PET scanner's detector array. (The machine at the bottom of this meme is a PET scanner.) So if you put a living thing in the PET scanner that has eaten or been injected with something that is undergoing positron emission, you can tell where in their body the position emission is happening.

This is useful because you can use chemistry to attach something that emits positions to something that a body will move to specific locations based on what it is. For example, if you want to find cancer cells: they absorb way more glucose than normal cells. So you can attach unstable fluorines to glucose molecules and inject them into a cancer patient to find the cancer.

Wherever you see way more positron emission happening than normal, that's where the glucose is going. So if we know cancer cells are absorbing glucose at an abnormal rate, now we know where in the body the cancer is.

(That's more or less how I explained my previous job to my 4-year old niece, but with more drawings and smaller words.)