As you asked in English, here is an answer in English.
The phrase "se la pasaba + gerund" can roughly be translated as "she spent her time + -ing", or "she kept + -ing". This phrase indicates that the action happened often, and during relatively long periods of time. It implies (sometimes exaggerating) that the action took up considerable chunks of the subject's time. For example:
- No tenía nada que hacer, y se la pasaba viendo vídeos en Youtube (She did not have anything to do, and she spent her time watching Youtube videos)
or, in present tense,
- Desde que empezaron a vivir juntos, se la pasan discutiendo (Since they moved in together, they keep arguing).
Note: the phrase "se la pasa(ba) + gerund" seems to me more common in (some countries of) Latin America. I am not used to hearing it in Spain, where it is more common to say "se pasa(ba) [el día/el tiempo/la vida] + gerund".
On the other hand, the phrase "estaba + gerund" is the standard past continuous tense, and describes what the subject was doing at a specific, single point of time in the past (similar to "was + -ing" in English):
- Cuando volví del trabajo, vi que ella estaba merodeando cerca de mi casa (When I came back from work, I saw that she was hanging around near my house).