¿Qué significa O sea ?

See a translation

It's used to correct, emphasize or define/clarify something.

I'll meet you in the city, that is, I will if the trains are running.

(Te veré en la ciudad, o sea, lo hare sí los trenes pasando)

I think "That is /that is (to say) is a good translation for that expression in english

Some people also tend to use "like" in the same context:
Ex:

My boyfriend left me yesterday because of my family, like, I dont even understand.

(Mi novio me dejo ayer por mi familia, o sea, ni siquiera entiendo.)


Even "so" works.

(phone conversation)

Person 1: The car broke down.
Person 2: so, you're not coming?

(Persona 1: el carro se descompuso
Persona 2: o sea, no vendras?

That's basically what we use "o sea" for