The Action is along the lines of casting a spell, moving, or something like that. This is the normal sort of activity that players of most RPGs expect in a combat round (although there is only 1 action per round, not 2 or 3 or 12).
Reactions are in response to a trigger event. For instance, trying to parry a sword attack, block with a shield, or halt an ally's fall over the cliff.
The design idea with Reactions is that a player can keep "playing" even when it isn't their active turn to play. Aside from granting various defensive counterplay, this also makes combat feel more involved.
Of course, it also opens the doors to effects or knowledge that allows extra Reactions in a turn. For instance, a master swordsman might be able to parry more than once in a round. This yields more of the cinematic style of combat that I'm trying to achieve.
Everyone is familiar with Haste-like spells like that grant additional actions in various systems, so why not magic for extra Reactions?
Here's an example low-level spell in the current draft, for those who wield Dimensional magic. The study of Dimensions is complex and esoteric, with powers over time and space itself.
Bullet Time (Dimension Support)
Difficulty: Very Hard (this is for learning the spell, not casting it)
Affects: Ally, Ranged 10 + INT’
You grant an ally a small reserve of temporal power. When this spell is cast, roll 1d4 + half-level. During this encounter, the ally gains that many extra reactions to use, at their discretion. They can use the extra reactions during a single round, or spread them out among several rounds. Once the reactions are spent, the spell ends.
You cannot cast this on any character more than once per encounter, but you can cast it on different characters within the same encounter.