When you take a photo, sometimes the edges of the picture look darker than the middle. This is called vignetting! It's like when you look through a little telescope and you see a circle around the picture you're looking at, except in a photo it's usually a blurry dark spot.
This can happen for different reasons, like if the lens you use isn't big enough for the camera you use, or if something is blocking the light going into the camera. But sometimes if you want to make your photo extra special, you can add a vignette on purpose! This means you make the edges darker than the middle on purpose, to make it look really cool.
So, vignetting is something that makes the edges of a photo look darker, and sometimes it happens by accident and sometimes we do it on purpose.