… what do they do?

You can “backpack” something from someone else's project like a sprite, music, etc. and un-backpack it in your project so it's basically a way to copy and paste something from another project. PS: If you choose to do this give credit to the original creator!

Okay… that's pretty cool!

Yeah. Its good for really long and complocated scripts.

a backpack is a thing in which you can store scripts, sprites ,and sounds . you can also store things from others projects.