Drama
The Spanish Tragedy is a drama. You know, a story composed for theatrical performance. We good with that? Awesome.
Tragedy
But it's also a tragedy. A tragedy is a dead serious work that follows the downfall of a heroic character. Most typically, destined-to-fall heroes are super capable characters of high esteem whose actions affect not just themselves, but entire kingdoms. And their fall is usually the result of a tragic flaw.
Here are some examples of tragic heroes and their flaws: Hamlet suffered too much wishy-washy thinking), Macbeth's problem was too much ambition, Oedipus was blinded by pride—and later blinded by blindness, and Anakin Skywalker/Darth Vader were too pissed about past events—and that theme song of his just makes doing bad feel so good.
You might argue that Hieronimo has a tragic flaw of his own (we dare you to), or you might think that he fits the mold of another kind of tragic hero: a hero in conflict with an overwhelming force. What's the force? Well, first stop thinking of Darth Vader because we're thinking of another kind of force. We're thinking of a force like a corrupt system, the will of kings, or fate as determined by the gods.
What's the Takeaway?
Once you find a tragic flaw or overwhelming force, much of the meaning of the play will come into sharper focus. This is because tragedies are meant to teach us a lesson. Take Darth Vader, for example: if he had put his anger about the past aside, his fall from grace would've never happened. Of course we also would've missed out on awesome light saber duels, Princess Leia's cinnamon roll hairdo, and Han Solo's devil-may-care heroics.
Okay, the point is that tragedies tell ordinary folks like us how to live well and avoid our own downfalls. What can you learn from a 16th-century Knight Marshal from Spain? Well, for one thing, you should probably have better hygiene than a 16th century European. We'll let you hash out the rest.
Back to the Roots
It's true that there would be no Kyd without Seneca, but The Spanish Tragedy has a vivacity that Seneca's closet dramas don't even try to achieve. And no, Seneca did not stage his plays in closets—a closet drama is a term for plays that are meant to be read, not performed. Because they are closet dramas, Seneca's plays read like a gathering of long, wordy speeches that ponderously moralize on tragic violence (yawn). Kyd on the other hand, uses Senecan over-the-top violence to create suspense, intrigue, and good old-fashioned entertainment.