Final answer:
The $group stage in MongoDB's aggregation pipeline groups documents by a specified field and outputs documents with aggregate values computed from the grouped documents. It is used for operations like summing totals or calculating averages within the grouped data.
Step-by-step explanation:
The $group stage in MongoDB's aggregation pipeline is a process that collects documents together for the purpose of performing certain aggregation operations. This stage groups input documents by a specified identifier expression and for each distinct grouping, outputs a document. The identifier is specified via an _id field, and the output document can contain aggregate values computed from the grouped documents.
For example, if you have a collection of sales data, you can use the $group stage to sum up total sales per region. The documents that are output by the $group stage would contain two fields: the _id field containing the region identifier and another field with the total sales amount for that region.
The aggregation can perform a variety of operations, such as summing up values, calculating averages, or implementing more complex accumulating operations.