Role creation for MongoDB in vault

I am new to Vault. I found the below role creation statement from https://learn.hashicorp.com/tutorials/vault/database-mongodb?in=vault/new-release. What does the below json mean? Is it “admin” db with role “readWrite” granting “read” role for the “foo” db?

{
  "db": "admin",
  "roles": [
    {
      "role": "readWrite"
    },
    {
      "role": "read",
      "db": "foo"
    }
  ]
}

Also while establishing connection from vault secret engine to MongoDb, should we always use “admin” db for the connection like below?
mongodb://{{username}}:{{password}}@127.0.0.1:27017/admin?tls=false