Learn Italian: Avete dimenticato le vostre chiavi. Ve le portiamo stasera. - You forgot your keys. We are bringing them to you tonight.

When you combine an indirect object pronoun like vi (to you all) with a direct object pronoun like le (them), the 'i' in the first pronoun changes to an 'e'. So, vi + le becomes the combined pronoun ve le.

What exactly does the le in ve le refer to?

It refers back to le vostre chiavi (your keys) from the first sentence. Because chiavi is a feminine plural noun, you must use the feminine plural direct object pronoun le to replace it and say "them".

Why is it le vostre chiavi? Doesn't chiavi look like a masculine plural noun?

The singular noun is la chiave (the key), which is one of the Italian nouns that end in -e. It is feminine, so its plural is le chiavi. The possessive adjective must match this feminine plural gender, so it becomes le vostre chiavi.

Why does the sentence use the present tense portiamo (we bring) for an action happening stasera (tonight)?

Just like in English, Italians frequently use the present tense to talk about firmly planned events in the near future. You could use the future tense (porteremo), but the present tense is very natural and common for things happening later today.