As Ale said, it is a neuter definite article referring to any thing ( the third person in its neutral form) so it refers to things or concepts. So, if you say: "lo que pasa es que..." you are saying "what happen is... "lo que me está molestando es..." you'll say "what is bothering me, is that... "lo que dije es..." = "What i said is..." So as you can see, is not that hard, just needs a little practice!

No todo lo que brilla es oro = No todo aquello que brilla es oro= No todas las cosas que brillan son oro. Otra versión de este refrán es: "No todo cuanto reluce es oro" o "No es oro todo lo que reluce" En esta página, si te interesa, puedes mirar "que" y "lo" : http://buscon.rae.es/dpdI/

I think this can help you understand: http://spanish.about.com/od/sentencestructure/a/lo.htm It is not an easy answer because you can use LO in many ways. In this case, it is a neuter definite article referring to any thing. The translation of this quote is "not everything that glitters is gold"