Se trata de often means
It's a case of or It's a matter of
and it functions to explain what something is really about, what the true essence of the thing is. Note, it is an idiom.
I looked in several dictionaries, both all-Spanish, and translating, and couldn't find it. I'm sure someone else will find it. I'm certain that when I was learning Spanish, it was treated as an idiom in the book I was studying from. It's really very strange that I couldn't find it today. I'll try to explain it as I understand it.
First, let's consider tratar de without the se:
Este libro trata del ascenso de un dictador | This book is about the rise of a dictator OR This book treats the rise of a dictator.
Do you see, this is a bit like This book is a treatise on such-and-so subject? Or This book is a novel treatment of such-and-so material?
The next step is:
¿De qué se trata la película? | What is the movie about?
This is an idiom. I can show you some expressions in English that have some little bit of overlap (treatise, treatment), but really, it's an idiom. In other words, it means something more or different from the simple sum of the parts of the expression.
Translating from Spanish to English will yield a ton of different results and this might be confusing for someone checking Linguee.com for this idiom.
What might be more interesting for someone trying to understand this idiom better would be to notice English text that would naturally get translated to tratarse de. Here are some phrases to watch out for:
- It's a matter of...
- It's about...
- We're talking about...
- The idea is to...
- The goal is to...
- It's a case of...
- The point is...
Time for a joke.
Una ancianita iba caminando por la calle cuando vio una bola de gente. Se acercó pero no logró ver ni oír nada. Entonces le preguntó a un muchacho, "¿De qué se trata, joven?"
El joven le explicó, "Se trata de una riña."
La ancianita, que no oía bien, dijo, "¿Una niña?"
El muchacho aclaró, "No, señora, una disputa."
La ancianita comentó, "Ah entonces, no era tan niña."
I will translate your three examples. To do this well I need more context for each of your quotes. (I found that all three quotes came from a book by C.S. Lewis called La Travesia del Viajero del Alba.)
I hope this approach helps.
Los súbditos de Bern, a gran cantidad de los cuales vieron trabajando
en los campos, eran todos hombres libres y se trataba de un feudo
feliz y próspero. (CS Lewis)
Bern's subjects, a large number of whom they saw working the fields, were all free men; and this was clearly a case of a (estate).
[...] ---Tenemos nuestra espadas, Señor ---dijo el ratón.
---Sí, Reep, ya sé que las tenemos. Pero si se trata de una cuestión de reconquistar las tres islas, preferiría regresar con un ejército un
poco más numeroso.
"We have our swords, sir," said the mouse.
"Yes, Reep, I know we do. But if it's a matter of [if we must treat this as a matter of] reconquering the three islands, I would prefer to come back with a slightly larger army."
[Alternatively] If what we're trying to do here is retake the three islands, etc. [Note, the Spanish translation here is a bit weird and redundant -- "Pero si se trata de una cuestión de reconquistar" is using two ways of expressing the same idea. Actually, the translator could have said just "Pero si se trata de reconquistar" OR "Pero si es cuestión de reconquistar"....]
[...] Pero de vez en cuando Lucy ganaba porque el ratón efectuaba algún movimiento ridículo como enviar a un caballo a una posición amenazada por un a combinación de reina y torre. Aquello sucedía porque había olvidado por un momento que se trataba de un juego de ajedrez y pensaba en una auténtica batalla y hacía que el caballo actuara como sin duda él lo haría de estar en su lugar.
But from time to time Lucy won, because the mouse would sometimes make a ridiculous move such as sending his knight into attack from a combination of queen and castle. That would happen because he had forgotten for a moment that this was a chess game [was about playing chess]. He would imagine the knight was in an authentic battle, behaving as he himself would do in his shoes.