It's a response to someone who has expressed their love for you, and it means you feel the same way back.
Here's a breakdown of the phrase:
* Te: This is the object pronoun, meaning "you".
* Quiero: This is the verb "to want" in the present tense. However, in this context, it expresses love.
* Mucho: This means "very much".
* También: This means "too" or "also".
Therefore, the entire phrase expresses a strong reciprocation of love.