Lo cual can only be used to refer to something that has already been mentioned in the same sentence.
Él siempre se jacta, lo cual me molesta mucho.
He always brags, which really annoys me.
Hay un libro nuevo por Isabel Allende, lo cual me emociona mucho.
There is a new book by Isabel Allende, which I'm really excited about.
Lo Que
Lo que can be used any time lo cual is used, and it can also be used to refer to something that has not been brought up before in the same sentence.
Lo que can also be used at the beginning of a sentence.
It's better to use lo que instead of lo cual when using non-specific pronouns such as eso, esto, todo, etc.
No puedo contarte lo que escuché en la escuela hoy.
I can’t tell you what I heard at school today.
Lo que dijo el presidente es importante.
What the president said is important.