How should I understand mongoDB GridFS?

How should I understand mongoDB GridFS?

I am now using perl script client to store some big data into mongoDB .But
now I met a problem ,some document exceeds the size limit of 16M,so ,I
have to choose GridFS.From the GridFS document ,I read this:
GridFS is a specification for storing and retrieving files that exceed the
BSON-document size limit of 16MB.
Instead of storing a file in a single document, GridFS divides a file into
parts, or chunks, [1] and stores each of those chunks as a separate
document. By default GridFS limits chunk size to 256k.
It really make me confused.What does it mean by "file"?"Instead of storing
a file in a single document",it means , mongoDB stores a file in a single
document without using GridFS,right ?But I think it should say:"Instead of
storing a document in a single file,...".So ,the relationship and
difference between "file" and "document" make me confused.