Significance of $$ in MongoDB

I was recently given this snippet of code that generates an arbitrary document in MongoDB. I've never encountered $$ in Mongo before and I'm curious about its significance.

All I know is that if I run the code with a single $, then the n no longer appears... Anyone have an explanation?

What is the significance of $$ in $$n

Code

db.collection.aggregate([  {"$limit": 1  },  {    // use $range to generate iterator [1, 2, 3]"$addFields": {"rg": {"$range": [          1,          4        ]      },      globalVar: 0.001    }  },  {    // do the mapping according to logic"$addFields": {"cte": {"$map": {"input": "$rg","as": "n","in": {            n: "$$n",            f1: {"$multiply": ["$$n",                20              ]            },            f2: {"$cond": {"if": {                  $lt: [                    {"$multiply": ["$$n",                        6                      ]                    },                    {"$multiply": ["$globalVar",                        100                      ]                    }                  ]                },"then": {"$multiply": ["$$n",                    6                  ]                },"else": {"$multiply": ["$globalVar",                    100                  ]                }              }            }          }        }      }    }  },  {    // wrangle back to expected form"$unwind": "$cte"  },  {"$replaceRoot": {"newRoot": "$cte"    }  }]);

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