Learn Italian: Camminiamo fra questi alberi.

fra translates to "among" or "between". In Italian, fra and tra mean exactly the same thing and are completely interchangeable. You could also say Camminiamo tra questi alberi and it would mean the exact same thing!

Why is the word for "these" questi and not queste?

Because alberi (trees) is a masculine plural noun (the singular is albero). questi is the masculine plural form of "these". If we were walking among feminine plural objects, like piante (plants), we would use queste.

Where is the word for "We" in this sentence?

It is built right into the verb! The -iamo ending on Camminiamo tells us that the subject is "we" (noi). While you could say Noi camminiamo, Italians usually drop the subject pronoun because the verb ending makes it clear.

Does Camminiamo mean "We walk" or "We are walking"?

It can mean both! In Italian, the regular present tense covers both the simple present ("We walk") and the present continuous ("We are walking"). The context will tell you which one makes the most sense.