What does lo siento mucho mean?

Te quiero comes from the verb querer, which is "to want", "to wish", or "to love". Te would be you, so it is most likely "I like/love you"

y means "and"

And lo siento comes from the verb sentirse, which is "to feel". So "lo siento" is "I feel for it", or "I'm sorry"

Long story short: "I love you and I'm sorry" is the most likely translation