How does HTML templating fit in with the backend language and database?
There are obviously many ways this can all work together, but it sounds like you have the (a) right idea. Generally the frontend deals with JSON, and the server provides JSON. What's consuming or providing those responses is irrelevant; you shouldn't need to worry that Mongo is your database, or that underscore is handling your templates. 
Think of your frontend and backend as two totally separate applications (this is pretty much true). Ignore the fact that your frontend code and templates are probably delivered from the same machine that's handling the backend. Your backend is in the business of persisting data, and your frontend in the business of displaying it. 
RE: Mongo using JSON/BSON; The fact it uses the same language as your frontend to communicate is a red herring. Your DB layer should abstract this away anyway, so you're just using Python dicts/tuples/etc to talk to the database.

I'm guessing i'd have to have a python script that pulls the information from the DB, create a JSON object, and use it to populate the fields in the html template.

Spot on :)
I apologize if this question is not specific enough, but I do need some help understanding this concept. I've been researching many Javascript libraries including JQuery, MooTools, Backbone, Underscore, Handlebars, Mustache, etc - also Node.js and Meteor (I know all those serve different purposes). I have a basic idea of what each does, but my question is mainly focused on the templating libraries.  
I think the general idea is that the template will be filled by a JSON object that's retrieved from the server. However, i'm confused by how that JSON object is formed, and if it can go the other way to the backend to update the database. Please correct me if this is incorrect.
For a more solid example, let's say I have Apache running on Linux, and am using MongoDB as the database and python as my primary language. How do all these components interact with the templating library and each other?  
For example, if I have an HTML file with a form in it and the action will be set to some python script; will that script have to retrieve the fields, validate them, and then update them in the DB? If it's MySQL I'd have to write a SQL statement to update it, but with Mongo wouldn't it be different/easier since it's BSON/JSON based?  
And for the other example, let's say I have a view-account.html page that will need to pull up user information from the DB, in what form will it pull the information out and how will it fill it into the template? I'm guessing i'd have to have a python script that pulls the information from the DB, create a JSON object, and use it to populate the fields in the html template.
I am aware there are web frameworks that will ease this process, and please suggest any that you would recommend; however, I'm really interested in understanding the concepts of how these components interact.
Thanks!!