The neuter article lo is used mainly before a masculine singular adjective or past participle, and sometimes before an adverb.

Lo importante es encontrar un buen trabajo.
- The important thing is to find a good job.
Lo bueno es que no se lastimó.
- The good thing is that he/she didn´t get hurt.
Lo atestiguado es lo que cuenta.
- What has been testified is what counts.
Lo hecho, hecho está.
- What´s done is done.
María sabe lo interesante que es el libro.
- María knows how interesting the book is.
Él no sabe lo difícil que es ésta tarea.
- He doesn´t know how difficult this homework is.