Introduction to Operators in MongoDB

These two filters look almost the same, but they ask different questions.

db.users.find({ age: 30 })             // age is exactly 30
db.users.find({ age: { $gt: 30 } })    // age is greater than 30

A plain value can only ask for equality. To ask for anything else, you replace the value with a small document whose key starts with a dollar sign. That key tells the server what to do with the value, and it is called an operator. Here, $gt means "greater than".

Operators appear wherever MongoDB needs an instruction rather than data