Learn Italian: Avevo lasciato la fotocamera sul treppiede. - I had left the camera on the tripod.

This is the trapassato prossimo (pluperfect) tense, which you are learning now. It is formed by using the imperfect tense of the helper verb (avere becomes avevo) plus the past participle of the main verb (lasciato). It is used exactly like "had done something" in English to talk about an action that happened before another past action.

Why is it sul and not just su?

In Italian, prepositions usually combine with the definite article (the word for "the"). Sul is a combination of su (on) and il (the). Because treppiede is a masculine singular noun starting with a single consonant, it takes the article il, giving us sul.

Treppiede looks like it's made of two different words. Is it?

Exactly! It is a compound word made of tre (three) and piede (foot), which translates perfectly to "tripod". Note that because it ends in an -e, you have to memorize its gender: it is a masculine noun.

Could avevo lasciato also mean "he had left" or "she had left"?