This tutorial is garbage. Not clear at all what instructor is doing nor why he's doing it.

I remember feeling that way also when I first saw this video, also.

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What is your background?

Have you worked with the Mean stack (MongoDB, Express, AngularJS and Node.j) before:

https://en.wikipedia.org/wiki/MEAN_%28software_bundle%29

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Without knowing what your specific "gap in understanding" is all I can do is post

a few supplemental tutorials:

http://www.sitepoint.com/two-way-data-binding-angularjs/

http://www.codeproject.com/Articles/808257/Part-Data-Binding-In-AngularJS

http://kodypeterson.com/angularjs-the-3-types-of-data-binding/

http://www.9lessons.info/2013/09/angularjs-tutorial-two-way-data-binding.html

http://sravi-kiran.blogspot.com/2013/02/EasyTwoWayDataBindingInHtmlPagesWithAngularJS.html

http://blog.hfarazm.com/data-binding-angularjs/

http://henriquat.re/intro/angular/angularjsForDotNetDevelopers.html

This one has a nice "Project Architecture" diagram:

https://codeforgeek.com/2014/09/two-way-data-binding-angularjs/

This StackOverFlow thread has some useful info on the "Digest Cycle":

http://stackoverflow.com/questions/30425052/how-angularjs-implement-its-two-way-data-binding-mechanism

A nice talk on comparing "the two-way binding in AngularJS, EmberJS and KnockoutJS":

http://2013.jsconf.eu/speakers/marius-gundersen-a-comparison-of-the-twoway-binding-in-angularjs-emberjs-and-knockoutjs.html

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Personally I not a big fan of the the AngularJS ng-model documentation:

https://docs.angularjs.org/api/ng/directive/ngModel

I did find this page helpful in increasing my understanding of the ng-model

(had a few diagrams):

https://masteringmean.com/lessons/293-Creating-Advanced-Forms-Looking-Inside-ngModel-Data-Binding

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The best thing I can tell you, Sua, is that Treehouse isn't the only source of information on any given technical subject.

You've got the most complete knowledge source that mankind has ever generated at your fingertips : The Internet.

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And generally what I recommend for students who find themselves getting "lost" after wandering into a technical "thicket" that is just too dense is:

Back out of the thicket and spend some time wandering around the outside of the thicket.

Circle around it to trace it's perimeter (and the technical subject matter surround it).

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Then come back to it after you've had a chance to explore around it from different "views"

and you may find the "thicket" a little less dense the second time around.