Here you will find information on Essence (the name for Magic in the Superunknown). If you're looking for more detailed info, the rules for Magic start on pg. 210 of the Core Rulebook.
Some important takeaways for using Essence:
If your character does not have at least one rank in a Essence skill, they cannot attempt to use Essence from that skill.
It's HIGHLY recommended, but not required, to take some ranks in Knowledge to compliment the use of Essence. Some additional effects will only trigger based on how many ranks of Knowledge you have.
One good guideline for Essence is that accomplishing something through the use of Essence should rarely be as easy as accomplishing the same task by using the skill designed for it.
When your character casts a spell that requires a check (whether or not the check succeeds), they suffer 2 strain after resolving the check.
There are three skills of Essence; Arcana, Divine, and Primal. Each one has access to certain spell types. You are however not restricted to only one Essence skill. Since Essence is of a very random and chaotic nature, those who wield it range greatly in strength, from those who can only control fire to those that seem to have no limit of Essence control.