What is a schema in MongoDB?

A schema in MongoDB is a description of a document's structure, but unlike in SQL, it isn't mandatory or rigidly fixed.

#A simple explanation for a junior level

1. There's no schema by default MongoDB doesn't require you to describe upfront which fields a document should have. The structure can differ from record to record.
2. The application defines the schema Usually it's the code that decides which fields to store, effectively defining the "data schema" that way.
3. A strict schema can be turned on if needed If control is needed, a JSON Schema is used, to check types, required fields, and rules.

Summary: a schema in MongoDB is an optional description of a document's structure. By default the database works with no fixed schema, but one can be introduced via validation if desired.