a schematic is a file that contains an area of blocks that you can paste into your world using a mod/plugin such as worldedit, an external tool like MCedit, or even place as a hologram in survival that you can then use as a guide to build the structure using the mod litematica. You can sort of think of it like a excerpt taken from a minecraft world that can then be pasted into another minecraft world. The main downside is that as far as I know there is no way to use it on bedrock edition.
---Hope this helped, Kitotek (UwU)

he did. he is so good!