"Frost" is a certain kind of finish that makes up the outer face of a titanium ring. It looks like the ring is covered in silver-colored frost (like things in your freezer, but the color of titanium), hence the name. But there are many rings which combine frost and other styles, too.
A Frost Titanium Ring
Check out these images of Frost rings to get an idea.
Frost is unique in that it can take the most punishment out of all the rings out there. It's mainly due to the textured finish, which is patterned in a way that makes scratches unnoticeable. As titanium is extremely scratch-resistant in the first place, a Frost ring virtually guarantees that it will be unaffected by even the most high-impact lifestyles. It's recommended for people who engage in activities like hiking, construction, contact sports, and so on.
How is Frost created? The Frost technique is a trade secret, and can only be achieved through handcrafting. Many rings you'll find come straight off an assembly line, but a Frost finish ring won't be one of them - so this makes it a recognizable indicator of quality. They're stunning to look at too; Frost rings sparkle in bright light, while in low light they are a subtle-yet-compelling sight to behold.