hehe, sounds like the title al-qaeda gave to the speech of bin laden.
Well, it means, literally:
"The word of the martyr of Islam -as we hope- to his Muslim nation"
Word by word break down:
كلمة = word (also used to denote speeches)
شهيد = martyr
كما نحسبه = as we think/as we hope. Religious phrase, often used because ultimately only God decides whether someone is a "true" martyr. Comes from "يحسب" meaning "thinks"; not in the sense of merely having a thought; that would be يفكر , but in the sense of thinking that something is true.
أمة (ummah) = nation; often nation does not refer to the population of a country, but rather it's a notion that transcends state boundaries, e.g. "Arab Nation".
لإمته (li ummatihi) = to his nation. To break it down:
لـ (li) = to, for
امته (ummatihi) is a conjugation of أمة "nation" that means "his nation". I don't know what's this rule called in English (sorry).