No, you can explain how it works in five minutes, given some background in data structures. (You cannot cover the details of how a particular filesystem, particularly a fancy one, works.)
It's a database that manages the allocation of fixed-size blocks on disk to files and stores metadata about those files. It generally has a header at a fixed position on disk that identifies the filesystem, stores filesystem-wide metadata, and contains a pointer (rather, offset and length) to the index of files. The index of files is a data structure (varies per filesystem; example: B-Tree) that stores a record per file on the filesystem. The record contains metadata for the file. Metadata varies per file system, but the key metadata stored is the collection of blocks on disk (and their order) owned by (allocated to) that file. Generally, every file gets a unique identifier and directories are implemented as lists of the unique IDs of files contained in the directory (plus, potentially, other metadata), though some filesystems implement directories differently.
Knowing too much about filesystem should not prevent you from being able to describe how they work at a high level. If it does, the problem is not knowing too much, but focusing too often on details in a context where the details are not warranted.