If you're not familiar with service, it combines Microsoft's Xbox Live Gold and Game Pass subscriptions into one package. The former allows you to play games online, while the latter grants you access to a library of more than 100 Xbox and PC games, among other perks. Some of the titles you can currently play on Game Pass include Gears 5, Halo: The Master Chief Collection, The Outer Worlds and The Witcher 3: Wild Hunt. The library of available games changes frequently, so there's usually always something new to check out.
If you're new to the service, your best bet is to take advantage of Microsoft's conversion offer. When you upgrade to Xbox Game Pass Ultimate, the company will convert any prepaid time you have on Xbox Live Gold or Game Pass toward your new subscription (up to a maximum of 36 months). You can combine this with the $1 Game Pass Ultimate promo by buying a prepaid Xbox Live Gold card. In this way, you can get 12 months of Game Pass Ultimate for $61. Just make sure to first add the prepaid time to your account before taking advantage of the promo offer.