It's a combination of the future tense of the verb "fazer" (to do) and the preposition "por" (for).
Here's how it works:
* "O": This is a contracted form of the personal pronoun "eu" (I) and the definite article "o" (the).
* "farei": This is the future tense of the verb "fazer" (to do), conjugated in the first person singular.
* "por": This is the preposition "for".
The entire phrase emphasizes the speaker's willingness to do something specifically for someone or something else.
Example:
* Portuguese: "O farei por você."
* English: "I will do it for you."