Translation:It is not hard to climb mount Kilimanjaro
Vigumu is the class 8 (plural of KI-VI) form of -gumu and also the adverbial form. As far as I've worked out, in Swahili, when an adjective doesn't have something to agree with, as in this case where there's no subject expressed, the adjective will be in the adverbial form. (Incidentally, it is the same in Esperanto, which probably helped me spot this pattern in Swahili.)
Another way you could say this sentence is Kupanda Mlima Kilimanjaro si kugumu. ("Climbing Mount Kilimanjaro is not hard.") In that case, the subject of the sentence is "Kupanda Mlima Kilimanjaro" and the adjective agrees with kupanda.