The closest English language analogue is basically "that which," but a smoother translation is usually "what..." as the start of an affirmative statement.
First example: "lo que pasa es que..." -> lit. "that which happens is that the boy..." and a smoother translation: "What happened is that the boy didn't go to school because he broke his ankle"
The reason that you can't just say "Que pasa es..." is that it sounds like you are either asking a question (without the proper accent) or wishing something in the subjunctive (but without the proper conjugation) for example: "que te pase bien"
The "lo" helps the other person understand that you are making a statement about a real situaiton.
Second example: "what he says he is a lie"
Third example: "what I don't understand is why she isn't hungry if she hasn't eaten since this morning.
Does that make sense?