What does the $in operator do in MongoDB?

The $in operator in MongoDB is designed to filter documents based on whether a field contains a value that matches any item within a specified array. When you utilize this operator in a query, you create a condition that allows you to select documents where the value of a specified field is found in the list of allowed values provided in the array. This is particularly useful when you want to retrieve multiple records that conform to any of several criteria without needing to construct multiple conditions.

For example, if you have a collection of "products" and you want to find all products that belong to certain categories, you could use the $in operator with the categories you are interested in as an array. This way, documents that have a category field value that matches any of those specified categories will be returned.

In contrast, the other answer choices describe different functionalities:

• Selecting documents with field values matching a single value refers to a more straightforward equality check using the regular comparison operators such as $eq.

• Counting documents with specified field values relates to the aggregation framework or using the count method, rather than the direct filtering that $in provides.

• Sorting documents based on specified field values is accomplished using the $sort operator in MongoDB, not $in.

Therefore, understanding the