A sundial portal is a portal that only opens at a certain time of day, and are found in every world.
They require Vexx to cross multiple platforming puzzles and obstacles to locate a Wraithheart. Each sundial portal is more difficult than the next.
If he falls into the abyss below, he will lose a life, and must start at the beginning of the portal once more.
As Vexx progresses through Astara, the portals become harder to find. And because the blue counters of the sundial get closer together, you'll have to be quick. Sundials are often used to make them open quicker.