The filter (https://docs.mongodb.com/manual/tutorial/query-documents/) is a JSON expression, which defines criteria by which documents are either included in or excluded from the final result set. The filter usually consists of two elements, a document field name and an expression. The expression can be a hard-coded value, a regular expression (https://en.wikipedia.org/wiki/Regular_expression), or a statement that involves one or more query selectors (https://docs.mongodb.com/manual/reference/operator/query/#query-selectors) (or operators).
A brief list of built-in query selectors is shown here:
| Operator | Meaning | Operator | Meaning |
| $eq | equal to | $and | logical AND |
| $ne | not equal to | $or | logical OR |
| $lt | less than |
