Vaccines are combat training for your immune system. They show it what to expect from a virus/bacteria so your cells can build a better defense when the actual pathogen arrives.

Usually, a vaccine gives your immune system dead or weakened pathogens so it can mount a specialized defense. Say an invading army is coming and you capture some soldiers and their weapons. You'll be in a much better position to repel the invasion than if caught by surprise!

mRNA vaccines, like Moderna & Pfizer/BioNTech's, work differently. They give your cells the blueprints for a piece of the virus, so your cells can build that piece (which, on its own, can't hurt you), study it, & be ready to attack it & the virus it's attached to when it shows up

It's kind of like instead of capturing the invading army's soldiers and weapons, you've got the 3D-printer files for their tanks and bombs and guns, and so now you can make them yourself, find every little weakness, and destroy them on sight if they actually arrive.

(If you have questions about the COVID-19 vaccines, check out this very comprehensive piece by @tarahaelle https://elemental.medium.com/amp/p/9a0eeb334ded)