@shibao imagine your filesystem, you say "make a snapshot", filesystem says "OK". Now something wants to delete or modify a file in it, filesystem complies but instead of deleting or modifying a file it makes a copy of it and works there instead, so previous version of file remains intact and accessible if needed, just within that snapshot, the rest of the system is none wiser. You could just access that snapshot, revert to it, remove snapshot entirely etc.

Say you want to make a backup of entire system while it is running, it's hard to do that because stuff keeps on modifying files, so you tell filesystem to make a snapshot, so all the writes happen in separate place, essentially letting you access filesystem frozen in time.