Can a Spanish person explain these to me???

One of the reasons Spanish is so hard to grasp is because Spanish speakers substitute things with pronouns without prior reference to what they're substituting. It seems that you have to be native to fully understand why some expressions use lo(s) instead of la(s) and vice versa. I have no idea what the pronouns are substituting in the following phrases:

1. Pasarlo bien
To what does the lo refer?

2. Las que lian
Las que?

3. Matarlas callando.
Las que??

4. Apañarselas