Cannot run my nodeJS app using localhost but it connects when I use an IP Address instead. What is the difference between these two addresses?

Question

I am trying to connect my mongoDB database to my app.js server but it says: error description

But it works when I change this line

mongoose.connect('mongodb://127.0.0.1:27017/wikiDB');

instead of:

mongoose.connect('mongodb://localhost:27017/wikiDB')

I was expected the server to be connected to database. Even though the database server was running it could not connect successfully.