El Pretérito Pluscuamperfecto also known as El Pluscuamperfecto and El Antecopretérito
El Pretérito Pluscuamperfecto is the past perfect tense of the indicative mood. It expresses an action that occurred before another action in the past: it could be called ''the past of the past'.
The English past perfect is formed with ''had'' + past participle, while in Spanish it's haber in El Pretérito Imperfecto + past participle.
Yo ya había viajado a Segovia cuando tu llegaste a Madrid.
I had already travelled to Segovia when you arrived in Madrid.
Nosotros habíamos visitado el Museo del Prado antes que tú.
We had visited the Prado Museum before you did.
See also: Spanish Past Tenses and All Spanish Tenses