There are 2 options for Stadia.
Stadia Base
Which is just Stadia, no charge to access the service. The caveat being you can only stream up to 1080p. You still buy games like you would in any other store front. The base version of Stadia isn't live yet but should be available to all sometime in early 2020.
Stadia Pro
Pro is a premium service on top of the normal version of Stadia. Unlocks the ability to stream in 4k. On top of buying games individually. You get access to select games you don't have to purchase - kind of like PlayStation Plus or the Xbox equivalent.
tl;dr
Having Stadia Base is like owning just the console. Having Stadia Pro is like having the subscription service those consoles offer.
@Phreakshow : The subscription to Pro gives you access currently to 2 games (Destiny 2 and Samurai Shodown) and additional titles will be available as time progresses, e.g. in December another 2 titles will be added. Additionally, the Pro subscription also gives you the ability to stream games at a higher resolution above 1080 and enjoy 5.1 audio. Plus, there is also the ability to save more video clips and access to some discounted prices for some games. So, if none of this is of interest then there is absolutely no need to purchase a Pro subscription, instead you can purchase games on Stadia and play them at no more than 1080p in stereo audio.
If however you were on a Pro subscription and had claimed some or all of the games that are on offer, then those games will not be available to you to play once your subscription has expired, if you want to play those games without a subscription then you would need to purchase them.
Let me know if you have any additional questions or if something isn't clear.
So 2 free games a month as long as I am paying? Thanks for the info. 3 free months and I can see where I stand at that point.
Sadly, disappointed then. Not what I imagined when I was excited about it.
Thank you!
@Phreakshow From help center, here's what included in Stadia Pro membership:
What's Stadia Pro?
With Stadia Pro you can play in up to 4K at 60 FPS with 5.1 surround sound and get discounts on game purchases. It also includes access to an ever-expanding library of games. You’ll have the best Stadia experience with Stadia Pro.
And it's not only two games, as more games will be added over time as part of Pro membership. It was announced yesterday that Tomb Raider and Farming Simulator will be added in December. It's a bit similar to how PS Plus works.