What does a field name prefixed with a $ signify in MongoDB?

A field name prefixed with a $ in MongoDB indicates a field path used to refer to the value in that field. This notation is commonly encountered in aggregation frameworks and query operations, where the dollar sign signals that the field is a reference to a field in a document, allowing operations to access the actual value stored within that document.

For instance, when using operators in aggregation pipelines, such as $group or $match, the dollar sign denotes that it is accessing the value of a particular field in the document. The use of the dollar sign helps differentiate between field names and literal values, making the query syntax clearer and more structured.

Understanding the role of the dollar sign is essential for effectively writing queries and utilizing MongoDB's features, particularly when dealing with operations that involve computations or transformations on document fields.