It is the past tense of the verb "passar-se", which means "to happen" or "to occur".
Here are some examples:
* "O que passou-se?" - What happened?
* "Passou-se algo estranho." - Something strange happened.
* "Não sei o que se passou." - I don't know what happened.
It's important to note that "passou-se" can also be used in a more specific sense, meaning "to be passed" or "to be transmitted".
For example:
* "A informação passou-se de boca em boca." - The information was passed around by word of mouth.
* "O vírus passou-se para o meu computador." - The virus was transmitted to my computer.
The exact meaning of "passou-se" will depend on the context in which it is used.