While I know that most seasoned, or even casual players, probably know what dndbeyond.com is all about I figured after spending several weeks of on and off viewing, asking members of my gaming groups, and also dropping some gold on the services, that it wouldn't hurt to put an explanation out into the expanse of the interwebs for other newbs like myself.
The easiest, most direct, and simplest way to describe dndbeyond.com is that it provides A) a wiki like version of the rules, aka a compendium, and B) searchable, sortable, hot linked tables (or toolsets) of things such as monsters, spells, and weapons.
It is a digital wiki and toolset of the D&D rules that you purchase from them and freely publishable content.
While preference would be the deciding factory, the idea is instead of buying the printed books and information on D&D you would purchase the rules and modules through dndbeyond.com and access them through the means described above.
Buying physical copies, from any location, of the rules or modules won't unlock them in dndbeyond.com.
I honestly wished I had figured out what dndbeyond.com was before I bought a bunch of books recently, but I can't say that I prefer it 100% over the books.
In the end though I will most likely purchase all content through dndbeyond.com going forward.
My only reservation, and this has been confirmed by an employee of the company; they currently don't have a plan in place for the content you purchased for if the company goes under. Meaning, while you are buying access to the content with individual purchases, if they were to go out of business tomorrow, we would lose access to those. They don't have a plan in place to provide some sort of offline access for the content in such an event. That still concerns me given how much money is being spent.
Their subscriptions are only for additional character slots and then also being able to share your purchased content with friends if they join one of your D&D Beyond Campaigns, which are basically groups.