Warning: If you do not want to know what boogers really are don't read this!
You may think snot (actually called mucus) is a really disgusting thing, but it is actually useful. If you breathe germs, pollens, dirt, and other things into your nose they would go down into your lungs. That would make it hard to breathe. That is where snot comes in. Snot stops the pollens, dirt, germs, and other things. After the snot stops all that stuff, the snot and hair in your nose (cilia) surround it. Then the snot and everything else hardens and turns into a booger.
