Topic first, then the comment
Spanish likes to put new information at the end. To say what you are talking about, you move that phrase to the front. This is called topicalización or dislocación a la izquierda (left dislocation): the fronted phrase is the topic, and the rest of the sentence says something about it.
Neutral
Leí el libro ayer.
I read the book yesterday.
A plain report; everything is equally new.
Topic first
El libro, lo leí ayer.
The book, I read it yesterday.
The book has already come up; only the when is new.
The pattern
What can go first
People as direct objects keep their a. An indirect object takes le or les. A whole idea is picked up with neuter lo. In writing a comma often follows the topic; after short topics like eso it is usually left out.
A María, la vi esta mañana en el metro.
María, I saw her on the underground this morning.
A person as direct object, with a.
A tus padres les mandé una postal desde Lisboa.
Your parents, I sent them a postcard from Lisbon.
Indirect object with les.
Eso ya lo sé.
That I already know.
Neuter lo for an idea.
A Lucía, el coche, se lo presté el sábado.
Lucía, the car, I lent it to her on Saturday.
Two topics, two pronouns.
What it is for
Left dislocation steers the conversation: it picks up something known, moves on to a new point, or sets two things against each other. In English it sounds casual. In Spanish it is common in speech and normal in writing, but it gets clumsy if every sentence starts this way.
La película la vi, pero el libro no lo he leído.
The film I've seen, but the book I haven't read.
Contrast between two topics.
¿Y el informe? Lo mando mañana sin falta.
And the report? I'll send it tomorrow without fail.
Moving on to the next point.
Las fotos de la boda, las tiene mi hermana.
The wedding photos, my sister has them.
Picking up what was asked about.
